Here is the full list of all 12 Outbreak: The Nightmare Chronicles achievements worth 1,000 gamerscore.
Defeat your first threat.
Defeat 20 threats.
Defeat 100 threats.
Defeat 200 threats.
Solve a puzzle.
Solve 10 puzzles.
Solve 50 puzzles.
Land a critical hit.
Land 10 critical hits.
Land 50 critical hits.
Heal yourself.
Heal yourself 10 times.