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