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Hmm mega blastoise with a modest nature no investment 2hko aegis lash with dark pulse so with a defensive set I believe u should still use mega and is a great asset to stall :)
setup sweepers such as thunderus and volcarona seem a big threat u could fix this by using a sdef heatran over ur current set
(Heatran) (M) @ leftovers
Trait: Flash Fire
EVs: 248 HP / 252 SDef / 8 Spd
Calm Nature (+SDef, -Atk)
- Lava Plume
- Roar
- Protect
- toxic