Here is the full list of all 13 Gunnihilation achievements.
Kill 10,000 enemies.
Collect a Combo Chain Shard
Clear the Jungle Lab mission on Easy Difficulty
Clear the Jungle Lab mission on Medium difficulty
Clear the Jungle Lab mission on Hard difficulty
Clear the Jungle Lab mission on Absurd difficulty
Collect a Combo Chain Gem on any Act after the first.
Kill 100,000 enemies.
Clear the Halloween mission on Easy difficulty
Clear the Halloween mission on Medium difficulty
Clear the Halloween mission on Hard difficulty
Clear the Halloween mission on Absurd difficulty
Kill 1,000 enemies