Just did this achievement the other day along with 'Actually, Hal, I can do that' (4 AI opponents on Hard or greater)
The 4 AI I played against were all set to Unfair. We were all on seperate teams with the teams locked.
I played on a large single star map, as a Vasari player.
Starting out the first thing I did was recycle my frigate factory and built a cap factory.
1st cap ship I buillt was an egg and sent it to the nearest astroid.
I went with Imperial labs to start with, my main focus was to get trade setup as fast as possible because the extra income will help in getting more cap ships out.
2nd cap ship was a carrier for the repair and added fighters.
3rd was a kortul
4th was a desolator
It was around my 4th cap ship that I had finally encounted the AI, and for a while it was touch and go. I had an early epic fight for a terran planet. The AI had about 20 frigates and a cap ship against my level 3 egg, level 2 carrier and both level 1 kortul and desolator. I would like to think that without the repair cloud I would of lost my egg.
I then pumped out two more carriers, keep in mind that all the strike craft I have built and will build will be fighters.
Next was another Kortul followed by a maurader for the speed boost, which is awesome btw.
My next 8 ships were all desolators, and with the phase missile updates... WOW! They do a ton of damage.
The game lasted for about 6 hours total. I saved often because every now and then I would crash (beta 2.5)
Once the majority of my cap ships were level 5 and 6 I never had to worry about any of them dieing. I eventually ended the game with my egg and 1st carrier at level 9 and the rest at level 8.
Near the end of the game, my total fighter fleet was between 42-48 fighters ( I can't remember the exact number) Those fighters were enough to keep the advent fighter/bomber spam from nuking any of my cap ships early in a fight. The weapons jam from the Kortul also helped a lot as well.
I would say doing this achievement with Vasari is the easiest due to phase gates. Cap ships can be slow to get around so having a phase gate keeps your fleet very mobile.
my2cents