Here is the full list of all 12 Defender achievements worth 200 gamerscore.
Shoot 5 mutants in one game.
Make it to the 2nd wave without losing any humans.
Achieve a score of 15,000 during an Xbox Live CO-OP game.
Win 30 Xbox Live versus games.
Rescue a human with a mid-air catch.
Make it to the 3rd wave.
Obtain a personal score of 15,000
Survive the first 75 seconds without shooting, bombing, or teleporting.
During an Xbox Live Co-op game, kill 5 baiters in any level without using any smart bombs.
Make it to the 5th wave.
Obtain a personal score of 25,000.
Lose no humans in the first 3 waves.