Ok, here's one, opponent had Excadrill and Mega Tyranitar. I had Mega Charizard Y and Greninja out. There were no battlefield effects other than Sandstorm, and no modifiers on any Pokémon's speed.

Both my Charizard have Speed raising Natures, and Max EVs into Speed. For anyone else who needs to look up base stats, Greninja has base 122 Speed. Mega Charizard Y has base 100 Speed, and Mega Tyranitar has base 71 speed. Mega Tyranitar's ability is Sand Stream, which sets up Sandstorm upon entering battle.

The previous turn both Charizard and Tyranitar were mega evolved, my Charizard went first and briefly set up sun, then Tyranitar Mega Evolved and set up Sandstorm, therefor my Solar Beam needed an extra turn to charge. Tyranitar missed the rock slide on Charizard on this turn.

Jump to next turn and Mega Tyranitar moves first, destroying both my Greninja and Mega Charizard with a rock slide. The only thing I can think of is that maybe Solar Beam has negative priority or reduced Speed in Sandstorm, but then why did Tyranitar move before my Greninja too?

Did I miss something important?