The picture below shows the race selection for this walk-through. I have a 70% success rate using this settings. You will loose if you have very aggressive neighbours and no good planets in the vicinity at the same time.

Master of Orion II race selection screen

Four phases to your success

You can split my strategy in rougly four phases. It really makes sense to do a hard cut between those and fully concentrate on your current goal. Just think about it. If you go to war you really should go the full way. Produce war goods like battleships and transports like crazy until the enemy is beaten. If you are low on command points just switch some planets to producing trade goods. That's how every country in our bloody history has done it. America switched most of it's factories from cars to airplanes. In the long run this is not healthy, but who cares after you won the war!

Now back to the phases. The first phase is beginning. You want to build up the technology, population and industry to produce colony ships as fast as possible. The key is to research the needed technologies as fast as possible. The technologies are reasearch labs, automatic factory, pollution processor and colony ship. First step is to research hydrophonic farms. This is a very fast research (50 RP) and immediately frees up one farmer for research.
Now if you have a good planet in your starting system build a colony base and have yourself a housing colony. In any case research labs then automatic factories and pollution processors so that you can get your colony ships going.

Battleship Your first battleships should still be armed with missiles. Such a battleship design as seen here is not very effective.

Phase two is colonization. You will never win the game if you have less population then the enemies. It's vital to have as many people as possible to stay ahead of the computer. Don't have any illusions. Even if you have the same population as a computer he will usually research much faster than you. Twice as much and you are on even ground.
The colonization should be done smartly. A colony ship is a big investment in production and should not be vasted on bad planets. A good planet is rich and/or terran. That might mean that you might have to look around for good planets. Usually the best planets are guarded by space monsters but that's not a big problem. I will describe tactics to beat them later on.
In a bad case you will not find good planets in the vicinity. Don't waste your colony ship even then. Build outpost ships to allow you to scout a wider area.

Two strange games come to my mind here. In one game I was literally surrounded by monster planets. Four of them near enough to be colonized. This was a big boost for me. I build a small monster fleet and moved to the first one (a planet with a splinter colony). In short order I had four perfect planets with natives and splinter colonies.
Another game had large terran planets but no rich ones. I was boxed in by two friendly alien races in the lower left corner. So I continued to research artificial planets and made my own planets. Some turns later I had only had six systems under control but with twenty planets. Most of them huge because I build them out of gas giants.

Phase three is build up for Orion. It is possible to win the game without a big population or without defeating the Guardian. It is impossible to win the game without doing either. Usually you will need Orion to speed up your conquest or even allow it. This page describes how to kill the Guardian with a very minimal fleet. Usually I go with at least three battleships or at least one titan.
Now, why don't I use the minimal fleet and instead use something that is at least three times stronger than recommended? The answer is: Usually you are not in range of Orion. The starting planets are distributed over the galaxy map and in most cases you will start at a edge out of range of Orion. In most cases an alien race is in the way and you will at least need to form an alliance with them. As the alliance has it's own problems I tend to go to war. Later on when I will talk about the example game you will see that I already have titans when I finally reach Orion. Not using them in this case is quite foolish. Avenger
The Guardian will fire first on the largest ship that can be killed in one round. So if you appear with ten fregattes and a battleship then the battleship will usually be targetted first unless it is armoured enough. The ship needs to have around 600 damage points to survive a round of combat against the Guardian. This is possible even with imperfect technology. Also, unless you follow the Guardian tactic exactly, the Guardian will be very effective in destroying missiles. The fool proof way is to set a point blank head shot. Your armoured battleship will use it's enhanced drive to fly directly in front of the Guardian and then shoot a small salvo of nuclear missiles to trigger the anti-missile defences. After that it will fire two salvoes of mirved merculite missiles using the fast-missile racks to clean the shields and then the emission guided merculites to explode the drive. All this must be done at point blank so that the missiles hit in the same turn.
A single titan can survive two rounds of combat against the Guardian if he has good enough armour and/or shields. In this two turns it can kill the Guardian with plasma cannons and archilles targeting unit. It get's easier with better technology. A single very high tech battleship can even kill the Guardian in one round before he can shoot back. For this you will need good creative tech, a good computer so you can start and achilles targeting, focus, autofire phasers or maulers, ...
So missiles are not a must and a Guardian kill is not required to win the game. However, if you had a bad start and the computer races are starting to get agressive you will need Locknar and his tech to beat them back.
And don't wait too long. If a computer player manages to capture Orion you will be way behind.

Phase four is the cleaning up. Orion is settled and you have at least as much population as the best enemy. However you can't sleep now. If you have only one big enemy you must hit him now. Use your Orion fleet (except one ship that needs to guard Orion) to show 'em what you think about cheating computer players that declare war just because you look a little bit weak. The fastest way is to destroy all the planets that you met and later on build some outposts for refueling. The slow way is to build transports and invade the better planets, which I mainly do because I have some issues with killing millions of inhabitants. If this weren't a computer game I would be forced to do the burned land tactic to eliminate the slightes chance of any alien race ever to come to my planets and do the "Independence Day" thing.
If you have two or even three big enemies (meaning they have nearly as much population than your own empire) then you will need to build up the population more. Starting a war now with one of you enemies could prompt the other ones to join in, leaving you in the middle of a two front war.
A better solution is to build up your population further until you have enough technology and resources to beat them all. In the worst case this is achieved only with the invention of the Star Gate which will allow you to instantly move your fleets back to defence after an attack.

Firing the stellar converter

Actually after I conquered Orion I never lost a game. Even the game where suddenly every enemy had Stellar converters was won after I realized that a cloaked ship could kill a planet bound stellar converter without any problem. Also using stargates I could move my fleets freely around using three doomstars to invade a planet and then moving them back the next turn to defend one of my own planets.

Game walkthrough

Now here is the eagerly anticipated game walkthrough. The settings are on huge, pre-warp galaxy with seven computer players. Then I designed my custom race as previously described. The starting planet turned out to be at the lower left quadrand however with some promising planets nearby. My own system has some small planets for housing colonies. I intend to build a colony base when I have nothing else to build. However I will try not to waste too much production on pollution. The plan is to have a research lab, automatic factory and pollution processor plus a housing colony before starting on building the first colony ship. At the beginning a colony ship is a huge investment. Start is at star time 3500.0 with 15 research points. I go for research labs.

3501.6: I have build my research lab and am now going for automatic factories. 35 RP

planet overview Home system planet overview. You see a quite good planet that I can use as a housing colony and later on (after having researched Terraforming and the gravity control thing) build up as a nice production planet.

3502.3: Automatic factories are researched and build. I haven't had enough production for the colony base yet, so I start now with researching freighters, building a the colony base and at the same time try to research pollution processor.

3503.3: Finally got the Colony base. Now I use the production to build a scout and go exploring while at the same time continue with the pollution processor. It takes some time because I still only have 35RP.

star time 3505.2 Galaxy map of the current status. You see the Space Crystal that I detected. You also see that I am again quite far from the middle of the galaxy meaning that I will need quite some time to reach Orion. A fast Guarding kill is out of the scope.

3505.2: A lot of things happened during the last twenty turns. Exploring the vicinity I found a nice planet to settle and another very good one protected by a Space Crystal which promptly destroyed the scout. I also hired two leaders.
Leaders are very good to boost research, production, spionage or combat. I usually hire every famous leader because he makes it much more likely to get other better leaders. I also hire every leader who gives production or science bonus. I even hired two of them once and switched between them when I wanted to produce something or do research.
For combat I recommend to improve your killer fleet with two or three leaders for the maximum punch. Navigation and ground attack bonus can make an attack a lot smoother.

3506.6: Finally I got my pollution processor. I go for biospheres now with still only 53 RP. Now phase two starts. We are starting colonization.

3506.9: Got my biospheres and now go for supercomputer. Still 53 RP and 18 Population. Clear target is to increase the population as fast as possible now.

3507.5: Finally got my first colony ship and also first contact. The alien is first bribed by giving him three bad technologies like scout labs and such. Never give away espionage technology. Also you don't need to give more than three techs so you can keep all the good stuff for yourself. I got a trade and research contract out of that.
Population is now at 21.

3508.0: I got a little bit nervous. Alien races tend to attack you if you look too weak. Being weak is mostly a factor of having not enough space ships. Also being in war with alien races makes you seem weak even when in truth you are fighting three races at the same time because you can beat them easily.
The result is that I will build up a monster killing fleet (three destroyers) and then grab the two nice protected planets. Afterwards you can decide to go on invading alien planets because a fleet is a horrible thing to leave laying around. So I go on researching battle pods and also building some spies. Currently at 48 RP and 22 Pop

star time 3509.1 History graph with the current population status. The population status is the only important metric to see here. Buildings don't tell you anything, fleet does not include the ships technology level which is a vital factor and the tech metric does seem to display very strange numbers indeed.

3510.0: Second contact. I have researched supercomputers and am still trying to expand as much as possible (as far as can be done when boxed in by two aliens). 57 RP, 28 Pop.

3512.9: Third contact. Now I have four destroyers and are going to kill the space monsters. The buildup might seem quite slow but it was actually quite conservative and safe. All new colonies get space stations. Mainly to get more control points, but also for defence and the ability to build battleships. Also the main target of this phase is still to colonize. For this the score is at 37 pop. As you can see in the picture it's lower than the Klackon population which needs to be changed as soon as possible. We need to beat them by at least 50% or they will outrun me with research and production.
Currently 134 RP per turn.

star time 3513.8 The history graph after having beaten the Klackons at the population game. It is vital to do this either by destroying the enemy population, conquering them or building up yourself. Hey, let's have sex :)

3517.0: During the last 41 turns I doubled my population. The count is at 72 Pop and 289 RP. This concludes the colonization phase. The new target is to prepare for Orion. For this I need to have emission targeting and fast missile racks. Note that until this time I have not detected Orion yet but I have a good notion where it could be hidden. Orion lies near the center of the map always.

3518.0: Just ten turns later my scouts finally detected Orion. It turned out that it's too far for ships without extended tanks which is common and very inconvinient. Now I have four options:

Current state is 323 RP and 82 Pop.

3520.1: I have build three missile titans. That should be enough to go against every planet and in border cases I could always retreat after killing the space station and then come back for the rest.
It turned out that three titans were on the weak side. The computer had missile bases with Zenon missiles and a very good armour technology. It needed almost two complete salvoes from the titans to destroy the base and at the same time I was hit hard by the returning missiles. At this time I have 604 RP.

Attacking the Guardian This picture shows how you should not do it. Actually I lost all titans just to make this picture. In the next Guardian turn it will trigger it's pulsar and destroy all the missiles. It will also destroy the titan it already targeted with the torpedo. You have to be at point blank range or have a very good timing with the missiles in order to avoid them being destroyed like that.

3522.0: All needed technology is researched and I have three titans ready to attack Orion. Again the force turned out to be sufficient but not exessive. I have 884 RP and 139 Population now.

And after Orion was defeated I re-equipped the titans with Death Rays and went conquering the galaxy. No computer was strong enough to stand in my way.
It might seem that attacking Orion was not required to win the game and you are right. In this game it just gave me some nice technology and a very nice planet plus a cool starship. I could have won without that by going to all around war.
However I played other games where the starting position was not as good by far. In such a game defeating the Guardian is required to win the game.

Attack on Antares An example attack on Antares. Again it's overkill. My England class Doom stars (all ships are named after countries in this game) kill everything in three rounds. After the lates patch the Death Rays have lost a lot of their benefits. Now the ship crew only get's killed when the rays go through the armour. This means that Antarian ships still have some crew left over and are very hard to capture. Before the patch you usually shot until all the crew was killed and then capture the empty ship.