ABOUT MONSTERS

Name

                Kill Weapon

       
Headless 
                        
         knife/guns

                               
Zombie
                                       knife/guns
Banshee

                      blue/orange/purple crystal(s)
Ghouls                                        knife/guns

                               
Ooz

                                                guns
Invisible Demon
                            guns
Nightmare

                                     guns
Electric Orb

                                   guns
Skitter

                                           guns
Sentinel

                                 green crystal
Flamethrower

                               guns
Ghast

    guns
Venomous Skitter                         guns
Greater Nightmare       
  guns (armour piercing)
Purple Monster
                         gas mask*
Green Ooz

                        none (get poisoned)

* Note:  It is generally believed that the gas mask does not help, though some have claimed that they were not damaged by the Purple Monster when they had it on.

POTIONS:

Clear Blue: healing
Bubbling Red: time slowing
Pale violet: temporary invulnerable/iron skin
Thick Brown: poison -> to get by the Green Ooz

CRYSTALS:
(Judicious usage of crystals is strongly recommended; i.e., don't use them unless it's absolutely necessary)

Yellow: Communication with dead soldiers
Blue: Freeze
Orange: Burn
Green: Earthquake (area damage)
Violet: Lightening (area damage)
Black: Turn to stone (not many uses)