Here is the full list of all 13 Ratz Instagib achievements.
Reach a rating of 250!
Reach a rating of 500!
Reach a rating of 750!
Reach a rating of 1000!
Win a match to unlock!
Win 10 matches!
Win 25 Duels!
Win 50 Deathmatches!
Win 50 Team-based matches!
Get a total of 30 Doublekills!
Get a total of 10 Triplekills!
Get a total of 5 Megakills!
Get a total of 3 Monsterkills!