MAKE SURE to have all your anti-alien treasures at near-100% for all chapters. Without that, these enemies will be very, very tough.
Luckily, no peons spawn during the boss battle. It's just the bosses. This is why single-target attackers like Macho Legs and Dragon will help.
Try to get rid of Bun Bun Symbiote quick; his shockwaves can end your army. He is definitely the hardest part of the level. Lead him nearby your base to avoid attacks from I.M. Phace and Galactic Overseer Nyandam.
Once you kill Bun Bun Symbiote, I.M. Phace should be easy. He doesn't have a ton of health. Once you kill him, just play the waiting game with Galactic Overseer Nyandam.
Or another strategy is Megaphrodite