Well, there are a few methods to try.

Drawing demi does work, if you have patience, but using just demi won't actually kill the boss, as it only continually halves the HP. You'll need to actually finish it off with an attack. I've found that generally, you have a successful draw 1 out of every 6 attempts, so it does take long.

Another method is to keep everyone on low HP and use the limitbreaks.

To be honest, it's more fun to experiment and see what works for you. Try a whole bunch of things, and you've eventually defeat him.

Take care all.