Renzokuken works but it takes much longer than Shot or Kamikaze. Every blow Squall deals with Renzokuken does about 2-3k damage on tonberry so there's a chance that he can counter with Everyone's Grudge (wasting more time) and there's also more hassle if you get Blasting Zone. Using Kamikaze on a character that has maxed out HP and reviving (Alexander) him in the next battle takes way less time.

So by using my methods the battle will be done in less than a minute but if you want to use Renzokuken or anything else mentioned in this thread go ahead