Modify your single player shortcut and add following text to the Target Field after all other stuff. (Plus symbol)set monkeytoy 0 (Plus symbol)set sv_cheats 1
Now start the game using that shortcut and hit ~ to bring down console and type one of the following cheats.
god-God mode
give health-Full health
notarget-Enemies won´t shoot at you
noclip-You can walk through walls
