Mappy is a very tricky game. There are patterns, but they are hard to execute.
Since you're not going for points here, don't worry about the order of the prizes to collect.
I basically work my way from right to left on all the boards.
Use the savestates and rewind if you get stuck.
Don't take too long on a level, because they will speed up.
Stay near the 'microwave' doors and lure as many enemies as you can into them.
Here's clearing level 19:
Sorry, this Game Clip has been removed