Got a third wave of cores coming through. This time, revolving around Mega Medicham:
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Surprised how this core isn't already mentioned, given Mega Medicham's incredible synergy with pivot support. Anyhow, Rotom-W is a solid partner thanks to Volt Switch in conjunction with its ability to scare out Tornadus-T and lure in Pokemon like Chansey and Ferrothorn that Mega Medicham will strike down in a heartbeat. Mega Medicham does have to tread carefully if Ferrothorn has Protect, though.
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Magearna pairs well with Mega Medicham for a number of reasons. First of all, its solid typing lets it serve as a defensive stopgap against targets like Choice Scarf Tapu Lele and Mega Alakazam. Once Magearna forces out a Pokemon that struggles against it, it can bring Mega Medicham in safely with a slow Volt Switch so that Mega Medicham can start wreaking havoc. This core works very well alongside Greninja providing Spikes support.
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Serperior's ability to bait in and punish Tornadus-T is a huge benefit for Mega Medicham, as this means that Mega Medicham can now pose a threat to Tornadus-T. Additionally, Serperior can blast its way through bulky Psychic-types that Mega Medicham struggles against, like Cresselia and Slowbro. In return, Mega Medicham can smash through Pokemon that attempt to check Serperior like Magearna and Kyurem-B.