Obviously the first thing to do is to beat the other monsters first so that you're down to just Windmaster Prime.

Colette is really susceptible to getting KO'd easily, so if you use her it's probably best to keep her back a bit. Marta will likely be spending all her time using First Aid so you're probably going to be down to yourself and one of your monsters (preferably a high leveled one or one with a lot of HP) dishing out most of the damage. I found that the best way to go at him myself was to just run behind and take him down one combo at a time. Windmaster Prime will usually cancel spell casting fairly easily and go straight into a guard, which makes it somewhat easier to avoid spell casting.

So pretty much, have Marta heal, keep Colette back (if you use her), attack with Emil and a high leveled monster, run behind, attack, guard and unison if necessary and you should be able to pull through.

It's also a good idea to be around level 15 or so.