Here is the full list of all 21 Guns, Gore & Cannoli achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Complete the game on impossible mode
Complete the game on hard mode
Kill 2000 enemies
Destroy 10 cars
Survive The construction site without killing any enemies with weapons.
10 multi-enemy grenade kills
Survive the river level using only your magnum
Complete the game on normal mode
Complete the game on easy mode
Kill 1000 enemies
Consecutively kill 25 enemies with a headshot
Defeat Mickey
Survive the sewer using only your Lupara
Survive the army stronghold without killing any enemies.
Complete 'Bonnino's Stripclub' without dying once on Normal Mode
Defeat Sonny
Kill 400 enemies
Kill 20 enemies in the alleystreets, during one run, by kicking them into fire.
Eat 10 Cannoli
5 multi-enemy grenade kills
50 headshots