Here is the full list of all 30 Killer Instinct 2 Classic achievements worth 1,000 gamerscore.
Perform an Ultra
Perform a Finishing Move
Spend 400 credits
Win 40 matches against the AI with Fulgore.
Win 40 matches against the AI with TJ Combo.
Win 40 matches against the AI with Orchid.
Win 40 matches against the AI with Jago.
Win 40 matches against the AI with Glacius.
Win 40 matches against the AI with Tusk.
Win 40 matches against the AI with Spinal.
Win 40 matches against the AI with Kim Wu.
Win 40 matches against the AI with Sabrewulf.
Win 40 matches against the AI with Maya.
Defeat Gargos with Fulgore.
Defeat Gargos with TJ Combo.
Defeat Gargos with Orchid.
Defeat Gargos with Jago.
Defeat Gargos with Glacius.
Defeat Gargos with Tusk.
Defeat Gargos with Spinal.
Defeat Gargos with Kim Wu.
Defeat Gargos with Sabrewulf.
Defeat Gargos with Maya.
Defeat Gargos with all characters.
Defeat Gargos using only one credit.
Perform a No Mercy on every character in the game.
Perform an Ultra with every character in the game.
See all Victory Animations and Endings.
Win a match in every arena.
Win 20 consecutive matches against the AI.