I haven't beaten the Pit of 100 Trails, so I can't give you much help other than to say be sure to bring lots of HP and FP powerups with you and don't even think of trying to beat it until very late in the game (either right before the final boss, or when you return to the city after beating the final boss.
However, I do know that if you quit at any time in the pit (either from dying or jumping in a pipe that returns you to the surface) you have to restart from the beginning. Also, if you haven't figured it out yet, there's no saving at any point. So be sure you have at least two straight hours to play before attempting it, otherwise you're just wasting your time.