Boss Fight- Red Skull
Complete the missions of the Evil Empire (excluding races and monument building) to get a shot at the Skull himself.
This fight goes in cycles- first you fight a bunch of Hydra goons. They're not tough, but the topography of the monument you're fighting on can make tracking them all down something of chore. Also, Red Skull will take potshots at you from the air, so stay in motion.
When you've disposed of the mooks, you'll be able to fight Skull himself, though a beam or projectile attack may be need to lure him out of the air. Pound him as much as possible until he summons more goons, and the cycle repeats.
Go through this three times to win the fight, and unlock Red Skull for purchase and use (ew). You'll also get a point toward Who's the Boss.