Misstep

Easy enough, mainly a matter of timing. But if B falls off one of the first movers, if he falls to his right he can just walk the wall and up the steps to the next section; if he falls to the left, you have to restart the level. Also, you have to get to the exit quickly, otherwise the zap bot gets stuck on the stairs, and B can't get past.

just wait at top for while for the exact time to make a step.