Earthworm Kim
Accumulate 1000 marbles and finish the game with a Smart Superhero rank to access Earthworm Kim. The difficulty of the game will be raised when you play as Earthworm Kim.

Reach the Fear part of the Brain and go in the door on the right of the Sacred Cow Of Contemplation. In this room is the path to the levels Poultryguyst and Poulrtyguyst 2. You should see a door being blocked by a metal gate. If you have 1000 marbles and have beaten Earthworm Kim, the gate will open for you. Walk through it and the game will reset. When you start the game again, Earthworm Jim hops around playing the accordion but this time Kim will be playing it and will soon be smashed by a cow. Then press any button to get to the file select screen. You should now have a fourth file as Earthworm Kim.

Beating Psycrow
Simply ignore the marbles, collect bullets always, and shoot him. Now keep his marble level at 0 or close to it at all times. This will allow you to achieve your marble level up roughly to 85 within a few minutes.

The Good, the Bad, and the Elderly: Easy Golden Udder
When your in the fantasy part of the brain, there is a building shaped like the back of a cow. Inside there you have to ride an annoying ice cube around a room. If you fall off, you have to start over. If you jump onto the ice cube and jump back onto the fridge, then you can just watch the ice cube slide around. Jump back onto the ice cube once it returns to the fridge but before it has stoped moving. You will receive a golden udder without doing anything.

Unlimited lives
Just go outside the entrance to "Hungry Tonight?" Grab the Power Parp Tin from Snott and use it to grab the extra life. Go into "Hungry Tonight?" and exit. Talk to Snott again to get a second Power Parp Tin, then collect the extra life once again. Redo this to get as many extra lives as you want.

Beating Fatty Roswell
Obtain two of the three missiles and wait at the third. Now he will teleport there and be an easier target.