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Actually if your running roost with skarmory you most likely wont need leftovers. Magnezone is commenly seen as it will almost always take down Skarmory, Scizor, and Forretress who are also seen frequently.
Try changing Skarmory's leftovers for a shed shell; it keeps you from being revenge killed by magnezone who would otherwise trap him. Try squeezing taunt in there by removing another move as many threats will be using some kind of setup. Trade Gengar's explosion for destiny bond; i doubt you'll...