Morokiane - I like your last message - it bleeds more confidence than my characters did blood the first time through!
However, the key to this level (which I previously found harder than any final battle!!) is clearing out all of the rooms on the way to Yxonomei. If you do not she releases the un-slaughtered minions upon you!
I must admit, my first time through, my characters were the same level as yours. The way I coped (eventually) was by summoning help, retreating my team (excluding summoned creatures), casting area spells, then closing doors behind me. Eventually you will be left with Yxonomei who is magic resistant and you whack her full on in melee.
My second time around, playing in heart of fury mode and expecting huge fatalities (though my average character level is 15-16) I whacked her dead without even realising it was her and without retreating back through the first door! LOADS of exp (180,000) at heart of fury level. (Yes I am gloating - but am privately pleased as I expected to have to reduce battle difficulty!)
The key is summon and summon and drop your main party to missile and area effect spells.
Happy gaming and keep up the battle cry - you will need it!