Here is the full list of all 45 KOP KILLER 22XX achievements.
Complete 3 Sewer Arenas in Arcade Mode in one run
Complete 3 Cell Arenas in Arcade Mode in one run
Complete 3 Lab Arenas in Arcade Mode in one run
Escape the WARRIOR brainwashing program
Complete your first Cell Arena
Complete your first Lab Arena
Complete your first Prison Yard Arena
Kill 50 Beat KOPs in Arcade Mode
Kill 50 Pistolero KOPs in Arcade Mode in a single run
Kill 100 Fascist Goons in Arcade Mode in a single run
Kill 50 Drones in Arcade Mode
Kill 25 foes by knocking them into hazards in Arcade Mode
Kill 100 Riot KOPs in Arcade Mode
Kill 50 Fascist Goons in Arcade Mode
Kill 30 Riot KOPs in Arcade Mode in a single run
Kill an enemy by knocking them into hazards in Arcade Mode
Kill 50 foes by doing fall damage in Arcade Mode
Clear 5 Sewer Arenas in Arcade Mode
Clear 5 Cell Arenas in Arcade Mode
Clear 15 Lab Arenas in Arcade Mode
Clear an Arena with full health and ammo
Kill 50 Beat KOPs in Arcade Mode in a single run
Kill 200 Beat KOPs in Arcade Mode
Kill 50 Pistolero KOPs in Arcade Mode
Kill 30 foes with ranged attacks in a single Arena
Kill 50 Drones in Arcade Mode in a single run
Kill an enemy with fall damage in Arcade Mode
Revive 10 comrades in Arcade Mode
Kill 15 foes with ranged attacks in a single Arena
Kill 50 foes while they are knocked out in Arcade Mode
Break 100 weapons by wearing them out in Arcade Mode
Clear 10 Arenas in Arcade Mode in one run
Clear 5 Lab Arenas in Arcade Mode in a single run
Complete 100 Combos in a single run
Complete 20 Combos at full health in a single run
Complete 10 Combos of at least 15 hits in a single Arena
Perform a 25 Hit Combo in Arcade Mode
Break 15 Combos of at least 4 Hits in a single run
Complete 3 consecutive, unbroken Combos in a single Arena
Find every upgrade in Act I
Clear every level of Act I
Kill Larry
Find your first upgrade