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