My wife is trying to fight the mark "White Mousse".
Many if not all strategy guides and helps suggest using berserk on the monster, but my wife can neither cast it on the enemy or even use bacchu's wine on it.
Even using reflect on herself and bouncing it at him doesn't work.

Any suggestions?

With the berserk, there is nothing to do but keep trying. The Bacchu's Wine should have a higher chance of sticking on him, but it is still tough to do this and will wear off with time. What I would suggest is buying or finding some armor that will absorb/nullify water damage and then spam him with fire spells. I would stay away from the Reflect method since he can absorb his own water spells and heal himself, though.

I've heard of some kind of "Viking" gear. Where can I buy this?

Btw, I figured out the whole Berserk thing, my wife didn't realize L1 was the button :P

The Viking Coat can be bought for 4000 gil in Rabanastre, Nalbina Fortress, and Mt. Bur-Omisace.