How to cut off limbs in single player for all the door challenges.
Guide to Cut Off Limbs
For your hand, use a firework and aim directly up, if you are aiming down a bit it will blow off your leg instead. I suggest doing this one first since it will require the most respawns
To cut off your head, use the spikes in the Zen Garden
For your arm (including hand) go back to zen garden and use those spikes again, stand under one and you can jump and have it just take off one arm.
For your feet, you can get them run over by the tram in the middle, as soon as you start to ragdoll, get out of the way and stand in front of it again until the leg you want falls off.
For a full leg, stand on a mine, i suggest doing this one last, be very carefull to make sure you step on the mine with what ever leg you want gone
I hope this helps some people and takes away the frustration of locked doors, unfortunately they do not stay unlocked, so explore them before you leave the game, do the pelvis room if there is one.