Xandar Boss Fight: The Vulture
When you've completed all the Xandar quests (not counting races and monument building), you'll get a heads up that the Vulture is ready to take you on. Fly to the BOOM marker to accept his challenge.
This Vulture fight is a lot like the one you had in the very beginning of the game, with one crucial difference- now you have characters that can fly. This makes it much easier to catch up with Vulture and give him the smacking he so richly deserves
However, very time you catch up to him, he speeds up a little. If you're having trouble catching him toward the end, try to out-maneuver him instead. He's confined to the same little loop of space you are- if he breaks left around a building, then you should go right, and ambush him on the far side.
Still, while this fight may be briefly annoying toward the end, you don't take any damage, and can't really lose. Just keep intercepting and face-punching until the Vulture is yours.