Master of Orion/Killing the Guardian

Something to note here that is completely unlike in Master of Orion 2 is that there is only one enemy in space that spawns guarding a planet. Master of Orion 2 has upwards of 10 on a given map and they expect these to be beaten fairly early in order to gain access to better planets in the mid-game.

In Master of Orion 1, the Guardian is the only thing guarding anything.

Also, it is very very difficult to kill early. It is a very strong ship and will tear apart most fleets that are not designed specifically to beat it. Even if ships are designed specifically to beat it they often have a really hard time doing so because the ship is just that strong. Another thing to note, it isn't really that important to beat the Guardian if you want to win most of the time. The only time it is often required is if you are in a losing position in the very late game and you can't research tech fast enough to dig out of the hole. Usually, beating the Guardian is just a status symbol more than anything else. Sirian posts lots of walkthroughs of games he has played on the Impossible difficulty setting and not a single one included defeating the Guardian as shown by his omitting it completely from his play by play analyses. Other pros often experience the same thing.

One other thing, Orion counts as a sort of Ultra Artifacts planet. Every time you beat the Guardian, you get the Death Ray, a weapon with a maximum of 500 damage per shot) AND you get 5 free random techs. Usually those are high level techs as well.  Random generated techs from Artifacts planets get you techs that are up to 10 levels higher than what you already have.  When you get to the stage where you can seriously think about beating the Guardian, you might be level 40 in some fields.  That means you could jump right to the highest tech in a field if you are lucky.  This can also get you techs that were not available in your tech tree.  This is a huge technological boon.

If you think you can beat the Guardian and you are behind some other races, it could help you catch up technologically.

The most cost-effective way to beat the guardian is to use exclusively small ships with beam weapons. He's pretty much immune to missiles so don't bother trying. The reason why small ships is the way to go is that the guardian's Death Ray and Stellar Converters are extremely ineffective against swarms of small ships, while if you use bigger ships all of his weapons will deal huge damage to you. His missiles deal huge damage to small ships, but you can dodge them completely by moving your ships back in the battle screen whenever he fires them, and they'll run out of juice. Eventually he runs out of missiles (he has only 5 volleys), and you can go manfight him.

Recommended small ship design on Impossible difficulty :
 * Class 6 or higher computer, class 7 recommended (guardian on Impossible has 9 beam defense)
 * 1 Mass Driver (Neutron Pellet Gun works on lower difficulties but does nothing on impossible against his class 9 shield)
 * Maneuvrability of 4 or higher, e.g Fusion engine without stabilizer (3 combat speed or higher, so you can dodge the missiles)
 * Nothing else. No armor, no ECM, shield etc. That's completely useless.

You need about 1500-2000 of these ships depending on design.

Alkaris can get away with significantly less (~1100). Mrrshans can do this much sooner because they only need to fit a mark 3 computer (if you think the Mrrshans suck, then consider they're the best race in the game for rushing Orion XD).

Again, dodge the missiles! Otherwise you get one-shotted :)