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I agree with this, HP Fire is the most optimum. imo the best partner for Serperior is Dugtrio with a voltturn core that baits in Heatran. For example I use Mega Pidgeot with U-turn to bait the tran
Mega Slowbro has also been pretty annoying with Ninja gone. Mega Slowbro and Mega Sableye make stall viable. You can't get hazards up with Mega Sableye on the field and Slowbro just needs a couple calm minds to destroy you with that nice natural bulk and its ability in that you can't crit him...
My bad about Mega Gallade, thought his base speed was 125. Your argument goes both ways though. Greninja cannot switch in without you sacking something. It's frailty makes it super hard to bring in and even if you revenge kill a poke with ninja it easily gets forced out by any of these pokes...
The thing with Charizard X is that it does so much that your opponent has to keep guessing. The three sets that Charizard can run basically changes its counters. A three attack+roost set is hard checked by Azumarill and bulky Landorus, but if Charizard runs Wisp then Azumarill and Landorus can't...