Here is the full list of all 39 Outbreak achievements worth 1,000 gamerscore.
Purchased 20 pieces of equipment in Onslaught.
Constructed 10 barricades.
Defeated 100 undead threats.
Cleared a wave in Onslaught.
Defeated 20 undead threats.
Purchased a piece of equipment in Onslaught.
Cleared your first scenario!
Cleared the Tutorial scenario.
Cleared all campaign scenarios.
Healed yourself.
Constructed your first barricade.
You failed to survive.
Defeated your first undead threat.
Successfully overcame a puzzle.
Heal yourself 20 times.
Heal yourself 200 times.
Clear 10 scenarios.
Clear 100 scenarios.
Clear the Outbreak scenario.
Solve 100 puzzles.
Clear the Chasmal scenario.
Survive 5 scenarios on Normal Difficulty.
Survive 3 scenarios on Hard Difficulty.
Survive a scenario on Biohazard Difficulty.
Survive a scenario on Hard or Biohazard Difficulty with one or fewer kills.
Clear the Miasma scenario.
Land 10 critical hits.
Clear the Enkindle scenario.
Survive the first five waves in Onslaught.
Survive the first ten waves in Onslaught.
Survive the first fifteen waves in Onslaught.
Survive the first twenty waves in Onslaught.
Survive the first twenty five waves in Onslaught.
Survive the thirtieth wave in Onslaught.
Construct 50 barricades.
Construct 100 barricades.
Purchase 100 pieces of equipment in Onslaught.
Purchase 200 pieces of equipment in Onslaught.
Defeat 1000 undead threats.