how about beat 1-9 with tofu boy? you have to use the glitch where you slide up a wall.
I almost got that :-/.
Also, the "Super Iron Man"
Do the game using any character you'd like (barring restricted chapters) and do this without dying.
1) Play order must be All light worlds (SANS Cotton Alley), then CH1-CH6 Dark World, Cotton Alley Light World, Cotton Alley Dark World
2) Levels in each chapter must be done IN ORDER
3) Changing characters is allowed
4) Resetting (if you can hit pause that fast) is OK.
Record how far you get and post it here! It's not really meant to be "completable", but it would be interesting to see how far you get!