Mega Charizard Y can pretty much win on its own against sand balance archetypes. Latios is pretty much the only check that these teams carry to it, and it gets trapped by Pursuit, which is pretty miserable. Tyranitar is a very nice check too, but it's very risky to switch it in directly due to Charizard's access to Focus Blast. The combination of Mega Charizard Y's ability Drought, high Special Attack, and access to powerful moves in Solar Beam and Fire Blast is just unfair.