This is done on the Boss level. Keep it on Medium difficulty and focus on finding a molotov cocktail and grenade early on. Use the molotov cocktail to get into the secret room and get the stun gun. Now, just keep stunning the boss and hitting him with your knife and grenade. Item spawn points will change each time you play the level, so keep that in mind. Also, use the various explosives around the room to deal large amounts of damage.