R21 and 33 are believed to be the hardest of the levels to complete.
It's extremely difficult to keep track of all the commands flying your way in R29.
R35 is easy enough until the absurd Zerg Rush at the end. You need to look up a guide just to figure out the pattern! ( L, R, R, L, R, R, L, R, L, R, R, LR) Don't think you can just whiff it and take the bad grade either. There's enough guys here to make you fail the game very easily.