To use the cheat codes, you have to edit a game file, so make it backup first! Use a hex editor to edit the "persist.dat" file which you can find in the game folder. Go to hexadecimal offset $44 and change the $02 or $06 value at that location to $07. Save the file and start the game. Now, press F1 to enable the diary screen and enter one of the following codes:
Press F8 to change characters: shapeshifter
Invincibility: liveforever
Ammunition and health: fatloot
Super ammunition: superammo
Regenerate health: elderrune
One-hit kills: touchofdeath
Cannot be knocked down: canttouchthis
Breakable objects: glassworld
Win level: winlevel
Lose level: loselevel
AI controlled characters fight each other: reservoirdogs
Gatling guns mode: roughjustice
Daodan power mode: chenille
Godzilla mode: behemoth
Fists Of Legend mode: fistsoflegend
Ultra mode: killmequick
Slow-motion mode: carousel
Big head mode: bighead
Mini mode: minime
Phase cloak: moonshadow
Weapons locker: munitionfrenzy