Thanks for the quick response!
I haven't been locking the boss pokemon into any moves so I am not sure what condition I am meeting. Maybe the answer for now is not to take so long to set up. If I memento, then use reflect or light screen (I currently only have 1 memento pokemon), then flash 6 times, then setup mega slowbro, that's 17 moves. I guess I should aim to keep it to 15 or less?