If these strats have already been found then I apologize for wasting your time.

Alternative Fire-Crab strat in 'Secrets'

From what I timed, this was just as fast as the other method. As for the strategy, don't go past the grate; start moving slightly before the fire-crab turns around (using the left one as reference). If done properly you should only need 2 flipendos for each crab. The obvious benefits here are that you're not exposed to the fire-crabs as long and it's simpler in terms of not having to flipendo them around the corner... not that it is difficult or anything, but sometimes they can get stuck and require more than 3 flipendo's, from my experience.

"Guillotine" strat in 'Corridor'

Not much to say here... Cast Skurge once you're ontop of the ledge and don't turn around... As long as your movement is good and don't cast Skurge early you should always make the cycle.