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