Here is a really simple and very lo-tech method of defeating the Guardian of Orion:
1- Design a ship with very little defence and lots of offence. The weapon in the first weapon slot must be able to bring down shields quickly, I usually use Heavy Graviton Beams. The second weapon must be Ion Cannons (Heavy mount if you still don't have version 1.31). Add as many of these as possible (focus more on Graviton beams)
2- Build as many of these as time and resources allow.
3- Go to Orion and kick some butt. Fire all your graviton beams first then use Ion pulse to disable the Guardian, the ideal end result is that you destroy it on your first turn.
Defeating any Repulsive race
If you have a run-in with any repulsive race early on in the game (especially the Silicoid), defeat them as fast as possible, otherwise their fleet will be too much for you. It helps to customize your race with the following: Cybernetic (to repair your ships during combat, like the Meklar; it hurts early in the game by taking 1 production and 1 food for every 2 colonists (rounded up) instead of 1 food/colonist, but is convenient and saves you from having to add the "Automated Repair Unit" special to your ships), Telepathic (this serves a 4-fold purpose:
1) it prevents other Telepathic races (i.e. the Elarian, and other races who customize themselves as telepathic when on Hard or Impossible mode) from taking your colonies over without Troop Ships.
2) it adds to your Spying bonuses (+15 to defensive i believe)
3) it allows you to Mind Control your opponents' planets so their colonists are immediately assimilated and you don't have to worry about building/buying an Alien Control Center.
4) it allows you INSTANTANEOUS control of any ship you capture from an enemy (although if they capture it back and you take it back again, you lose control of the ship for that combat).
If you cannot defeat their fleet, go directly for the kill *IF* you can; take over the planets so you have a jump-off point in case another fleet arrives and decides to blow out it's own race on that planet. Once you take out enough planets, the main fleet will deminish in size due to not enough resouces to command them.
Also another way to easily defeat your opponents is to have a colony at a wormhole. if your opponent has a colony at the other end of the wormhole AND cannot reach you at any other colony, you are safe from attack until your opponent acquires fuel cells that can reach you. For some reason, your opponents will not use the wormholes to their advantage, unless they are already sitting on a colony with a wormhole and have access to one of your colonies which is the only one they can reach (which is usually one you just took over).
Defeating the Guardian
The quickest way to defeat the Guardian at Orion is to have the Plasma Cannon. Refit or build either Battleships or Titans with nothing but Battle Pods, Damper Field (if you've already had run-ins with the Antarans), Reinforced Hull (Achilles Targeting Unit ignores armor), and as many Heavy Plasma Cannons that can fit (8 to 12 is possible if you've acquired Megafluxers). 3 or 4 ships is all that is needed; Guardian will have a "warp core breach" (large, blue explosion) on the second or third ship. Keep doing this until you have the desired Tech. Don't forget to save a space in your Ship Leaders for Loknar, as he has excellent stats (Ship Officer Dalan has the same stats, but without the Galactic Lore)
The Ideal tech to get off of Orion is the Black Hole Generator, Reflection Field, and the Spatial Compressor (the first 2 because you can only get them off of the Antaran Titans, and the third you can get off the cruisers), and the Death Ray (which you get all the time from Orion anyway, but can only be gained off the Antaran Battleship or Titan, not before)
Specialty Codes
Money(1000bcs)-hold Alt and type on the keyboard -moola
All Tech-hold Alt and type- einstein
See All Planets and Enemies-hold Alt and type- iseeall
All Races Attack You-first go to the Race Selection Screen, hold Alt and type- canonboly1.