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