Here is the full list of all 11 Battle Kid: Fortress of Peril achievements worth 1,000 gamerscore.
Unlocked the Good Ending
Beat the Super Mech
Beat Zedd
Beat the Amethyst Boss
Beat the Seahorse Boss
Beat the Nagaconda Boss
Beat the Cleo Boss
Beat the Chester Boss
Beat the Owlbot boss
Beat the Lotus boss
Beat the Minimech boss