Here is the full list of all 14 Outbreak: The New Nightmare achievements worth 1,000 gamerscore.
Restore your health with a healing item.
Heal yourself 25 times.
Clear a wave in Onslaught.
Clear five waves in Onslaught.
Clear ten waves in Onslaught.
Fail to survive.
Kill an undead threat.
Kill 100 undead threats.
Kill 1250 undead threats.
Land 20 critical hits.
Clear 5 non-tutorial campaign scenarios on Normal difficulty.
Clear 5 non-tutorial campaign scenarios on Hard Difficulty.
Clear 3 non-tutorial campaign scenarios on Biohazard difficulty.
Clear a non-tutorial campaign scenario on Nightmare difficulty.