Since nobody is posting sets anymore, I'll nominate again then (not because I'm greedy but because I really hate Greninja)
Name: Swampert
Status:
Counter
Swampert @ Swampertite
Ability: Torrent
EVs: 104 HP / 248 Atk / 4 SpD / 152 Spe
Adamant Nature
- Waterfall
- Earthquake
- Ice Punch
- Rain Dance / Superpower
Mega Swampert avoids the 2HKO from any of Greninja's moves, outspeeds Greninja and OHKO it with STAB Earthquake . Waterfall is a reliable STAB move that also gets boosted by the rain and is Mega Pert's strongest option against neutral targets. Ice Punch is used to hit Dragons such as Mega Sceptile, Lati@s and Dragonite. Finally, Rain Dance can be used in the last slot so that Mega Pert can replenish it's own rain to reduce the pressure on Politoed to come in too often. Alternatively, Superpower can be used to heavily damage Ferrothorn which is a pain for Rain teams to deal with otherwise.
Calculations:
212 SpA Life Orb Protean Greninja Ice Beam vs. 104 HP / 4 SpD Mega Swampert: 144-172 (39.2 - 46.8%) -- guaranteed 3HKO after Stealth Rock
40 Atk Life Orb Protean Greninja Low Kick (100 BP) vs. 104 HP / 0 Def Mega Swampert: 130-153 (35.4 - 41.6%) -- guaranteed 3HKO after Stealth Rock
244+ Atk Mega Swampert Waterfall vs. 0 HP / 0 Def (non-Water type) Greninja in Rain: 331-391 (116.1 - 137.1%) -- guaranteed OHKO
244+ Atk Mega Swampert Earthquake vs. 0 HP / 0 Def Greninja: 276-325 (96.8 - 114%) -- guaranteed OHKO after Stealth Rock