Here is the full list of all 15 Zero-G Gunfight achievements.
Get two or more kills with one RPG rocket
Kill someone with a bounced railgun or whacky gun shot
Die from collision damage
Shoot a corpse
Self destruct with the RPG
Complete the Syrup Stockpile Time Trial level in under 12 seconds in Time
Get 2 or more kills with only 1 Linear Rifle shot
Kill someone with the last bullet from a minigun
Stop a bullet with a bullet
Win a round with less than 10 hp remaining
Have a round end in a draw
Kill someone by colliding into them
Get over 100 kills in arena mode
Complete every time trials level
Deal over 100 damage in one shot