This scenario is quite simple. You just need to destroy all the enemies. I think Ferakia is trying to get to something - perhaps that blue crystal type object in the top room to the left, so I blocked it off, but I'm not too sure.

Either way, all the enemies (save Ferakia) come straight out to attack you. One of the Succubus groups follws her around, whilst the Sabre Tiger and two Sucubi near the bottom charge to the west to attack. Once you reach a certain point (nearest the north-end of the large pool), the Sea Serpent will attack. It's quite tough, so there's two options - take in loads of water units, and weaken them with archers (Nixie would be ideal), or try and coax them onto land - then weaken them with archers.

Once everything else is out of the way, concentrate on Ferakia herself. Lots of Holy spells and Trrops would be helpful, but not critical. And don't forget the Magical Bikini in the chest to the top - it's not very good, and only worth 220P, but it's fun all the same ^_^ Simply stand a General in front of a door to open it.