Drought will cut the power of water type moves by 50%, but mega groudon will still be 4 times weak to water, so he will go down regardless.

I hope Primal Groudon gets something to counter its massive Water weakness. Maybe some special ability like 'Evaporation' that negates Water attacks or at least reduces it. It's cool it at least gets a way to abuse Drought enhanced fire attacks, but yeesh…. Fire/Ground is pretty bad