Ok so am currently trying to catch Groudon. However, when it tries to use solarbeam it always takes 2 turns. Groudon's ability is drought so my question is: Does solar beam only execute in 1 turn in extremely harsh sunlight as I know it does with Desolate Land, or does it only work in 1 turn if I have caught the Pokemon? I.e. Mega Charizard Y will solarbeam take 1 turn? Sorry if it is unclear.
drought only works for the first five turns, while desolate land does make solar beam occur in one turn. I know this because my Groudon has solar beam and I tested it. If you've been battling it for more than five turns, since it can't go primal, it shouldn't be able to use solar beam in one turn.
Solarbeam should attack in one turn since any kind of sunlight on the field (or if you are holding a power herb) makes solar beam attack on the first turn.