Here is the full list of all 67 Knights & Guns achievements.
Finish the prologue
Kill an enemy with a blast from a Bomb
Finish Act 1
Finish Act 2
Finish Act 3
Finish Act 4
Finish Act 5
Finish Act 6
Finish Act 7
Finish Act 8
Finish Act 9
Finish Act 10
Finish Act 11
Finish Act 12
Beat the final boss
Beat every level in the campaign
Get all of the golden bullets for all the levels in the campaign
Kill 1191 enemies with pistols
Kill 1314 enemies with shotguns
Kill 1410 enemies with machineguns
Kill 1415 enemies with sniper rifles
Kill 1187 enemies with energy weapons
Kill 1241 enemies with explosives
Kill 1097 enemies with stream weapons
Kill 732 enemies with the flamer
Fire each gun at least once
Kill 5 enemies with a single explosion
Kill 3 enemies with a single shot (Dragonflies do not count)
Defeat a boss with a Forcegun
Beat a level without missing a single shot
Kill 10 enemies with a single Hammer Strike
Kill 50 enemies with Dash
Kill 100 frozen enemies
Fire 10000 shots
Die 10 times
Buy 1 extra life
Have 5 extra lives
Buy a book
Have all armor
Buy everything
Spend 10000 gold
Be served by a cat
Kill 10000 enemies
Kill 13 ghosts
Kill 10 Parachuters before they hit the ground
Kill an Airship
Beat 30 levels in single player mode
Beat 30 levels in the co-op mode
Get a gold trophy in the Endless mode
Get gold trophies on all levels in the Endless mode
Beat a highscore in the Endless mode
Play every level in the Endless mode
Survive 3 waves in the Endless mode
Fully complete Act 1
Fully complete Act 2
Fully complete Act 3
Fully complete Act 4
Fully complete Act 5
Fully complete Act 6
Fully complete Act 7
Fully complete Act 8
Fully complete Act 9
Fully complete Act 10
Fully complete Act 11
Fully complete Act 12
Beat all side levels
Get all golden bullets on all side levels.