Here is the full list of all 20 Guns, Gore and Cannoli 2 achievements.
Defeat the end boss
Kill 500 zombies
Have 20 enemies killed by propelling airvents in the 'Secret Base' level
Defeat the Tank boss
Destroy 15 vehicles
Simultaneously kill 5 enemies with the rocket launcher
Kill a German soldier by dropping the churchbell on top of him in the 'Killing Fields' level
Kill 100 enemies with the chainsaw
Defeat the German Commander Boss
Kill 200 German soldiers
Finish the Thugtown level, using only the pistol
Survive the Raid using only the baseball bat
Defeat the U-boat
Kill 200 zombies
Eat 50 cannoli
Kill 5 ball-and-chain zombies in mid air
Kill 100 zombies
Consecutively kill 20 units with headshots
Kill 150 Mafiosi
Kill 75 enemies with headshots