Here is the full list of all 23 Dementium II HD achievements.
Kill a boss with the nail gun.
Kill 20 non-boss enemies with the shotgun.
Kill a boss with the rifle.
Kill 12 non-boss enemies with the nail gun.
Broke all the breakable doors.
Kill 10 non-boss enemies with the flame thrower.
Kill a boss with the pistol.
Kill the malatesta.
Kill 10 non-boss enemies with the bone saw.
Kill the Collosus.
Kill 20 non-boss enemies with the rifle.
Kill 5 non-boss enemies with dynamite.
Kill 10 flying enemies using the sledge hammer.
Kill 25 non-boss enemies with the pistol.
Kill any boss with the shotgun.
Kill the Gorgamesh.
Kill 50 non-boss enemies with the shank.
Kill any Boss with the sledge hammer.
Kill the Wendigo.
Kill 15 non-boss enemies using the sledge hammer.
Complete Chapter 1 using only the shank.
Kill the Gnaw.
Killed your first enemy!