First rule of thumb: Concentrate on the prince and your offspring because the two of them can call other Rapax in magically like a 'summon rapax' spell. Okay -- make sure you've got your buffs up (Enchanted Blade, Magic Screen, Armorplate, Missile Shield, etc) going into the battle and then you need to have your wizard cast Elemental Shield and your Priest cast Soul Shield the first round while your other characters all do range attacks on the Prince (or if you manage to get within melee of him before combat starts, do that instead). If that monk of yours isn't RFS, have him casting spells to produce insanity in your opponents and if he can do damage to them as well, great.
The buff spells will keep your characters alive through the numerous fireballs and arrows sent your way.
Once the prince is dead, concentrate on the demon-child. Keep your priest busy alternating between Heal All and Rest All every round while your wizard casts as many incapacitating spells on the opponents as he can -- not damage spells so much as nausea, blinding, paralyzing, etc.
If you don't have many range attackers in your non-spellcasters then you'll be taking every third round or so to run from one group of monsters to another. ALWAYS RUN because that way you might get an attack or two in before the end of the round if the groups are close enough.
If you have a choice between a close group of archers and a farther group of magic-wielders, ignore the archers. In fact they should be your last priority.
As I said -- a lot of this depends on whether you've got decent ranged abilities -- your ranger could save you a lot of trouble. In fact, no matter what group you are near, always have him targetting someone in your next group you are going to be headed towards because if he can get any insta-kills you've saved yourself some hassles. As far as your samurai is concerned, you'll probably want him meleeing but if you need his spells to do some more incapacitating on the opponents, then let your fighters and rogue carry the melee burden.
It IS winnable, though for me it was the toughest battle of the game. Fortunately the Rapax don't go in for death cloud, quicksand or summon elemental. If -you- have any of those, however, they can really help you out. Quicksand and Asphixiation can take out good chunks of your opponents.
Good luck!