it's a little messed up how you lose the frost power whenever you miss a note when the frost notes are on screen, even if you're not playing that section yet. also, the combo should go up every 10 notes. not x2 at 12 and x3 at 46. This game is hard enough to play in rockstar mode.
Grab a chainsaw
enter gun store while chainsaw is active
buy a weapon (i got dual AK's at this point)
when you leave the gun store you'll have your new weapon.
pick up another chainsaw
you now have infinite chainsaw.
don't pick up any other item or enter a store or you lose it.
You can cancel PSOD by pressing Shift + C, however, you will not receive any rewards.