Here is the full list of all 12 Nexuiz achievements.
25 headshots
Use 25 Dynamic Mutators in a match.
Played a match on every map
Killed 3 in a row using melee attacks
Won your first game online
Use a CTF Dynamic Mutator to aid your team!
Kill a member of IllFonic or someone who has already won this trophy
You read the manual
Taken bot training
Use a dynamic mutator for the first time
Max out medium cost mutator
Awarded 50 unique medals