Here is my tactic for the fiend..... I have 1 serious damage-absorber currently in my party. That has to be either Morte with his resistances, or Dak'kon, using shroud of shadows AND blur AND sometimes shield on top of his ultimate blade, with this you can easily reduce his armour to around -15. One way or another, the damage sponge should have plenty of spare healing items, just in case. Next, with damage sponge in place, I have Dak'kon, whether he is playing damage sponge or not, cast Zerthimon's Focus on himself, and the NO casts scripture of steal, though Dak'kon could do it just as easily. Now Dak'kon simply beats the crap out of the fiend, with a 90% chance to hit and another 70% or so chance for a crit. Oh yeah, you can toss in an enoll eva's duplication on him for twice the hits as usual, and everyone else can attack too, but you'll find their damage and chances to hit somewhat lacking against this uber-fiend.
And just out of curiousity, can anyone find the fiend's stats and compare them to TTO's stats please?