>>52882820All it takes is sun. Rain (Dance) is mostly used on weather teams, rarely in fat balance or stall teams featuring Blissey.
I say rarely, Rain Stall is a nice option but you don't face them really often.
Then it takes just one opportunity for the fish to switch in and force the opponent's pokemon out. Proceed a Nasty Plot, click Tera on the only special wall who will try to stop your fish and you get what I calculated.
I'd agree with you if you let me boost the fish five turns. I'd rather say: Play smart and don't give the fish an opportunity to boost this one single time.