We can build a fighter for 18BC with BC4, Fusion, Zort armor, maneuver 4,MB cannon.
The guardian has a pretty respectable beam defense on Impossible. We need as good a computer as possible in order to be able to break its advanced auto-repair. I'll work on that before designing a fleet.
So on my turns I trade around to obtain every computer, propulsion, construction, and weapons tech I can find. I suppose obtaining more planetology tech would increase our income from planetary populations a bit but for the most part I don't bother with that. I do, however, find the Sakkra are willing to trade Advanced Soil Enrichment for High Energy Focus, so I pull the trigger on that deal. I also rebalance our tech spending away from weapons, which won't help much more for miniaturizing megabolt cannons right now; the main space requirement for those is coming from the power required, which needs propulsion tech to miniaturize. Instead I dump spending back into construction (for more space on ships,) computers, (for RC VI) and planetology (for terraforming +100.) We do manage to pick up Universal Antidote and Particle Beams on my turns; terraforming +100 and Pulse Phasors are next.
At the end of my turn we are now able to design a small with BC VIII, Duralloy, Fusion Engines, Megabolt Cannon, and Inertial Stabilizer. A couple thousand of these should be sufficient to kill the Guardian.