Here is the full list of all 23 Alien Blitz achievements.
Kill 100 monsters
Kill 3000 monsters
Reach experience level 30
Reach experience level 70
Complete secret level
100% completion for 5 maps
100% completion for 20 maps
100% completion for all maps
Complete game
Complete game in nightmare mode
One weapon at level 20
6 weapons at level 20
100% completion for episode 1
100% completion for episode 2
100% completion for episode 3
No damage taken during any map of episode 1
No damage taken during any map of episode 2
No damage taken during any map of episode 3
Complete game very quickly
Complete game in Hell mode