OK, so you’ve got a few good options there. I’d think about putting Triton in the mix, at least. He hits hard, and his healing buff can boost the effects from lesser healers like Boldtusk and Kiril. I also like Gormek a lot and used him on defense for a long time. With his beefiness, you can usually count on him to fire and give you that six turns of def down. Only problem is having to switch out Boldtusk, who I also like on defense a lot, especially with the revive skill. I don’t have Jack O’Hare, but a self-imposed defense debuff seems less than ideal, even with a hard-hitting AOE attack. In your case, you could switch out Melendor for Caedmon, though, for some extra oomph. You wouldn’t lose the dispel, and you have enough healers already.
I might think about something like Caedmon - Rigard - Li Xiu - Boldtusk - Triton. Fast snipers on the wings, hearty healers on the flanks, and a tank with an AOE mana cut seems like it should work pretty well in theory. In practice, I can’t make any promises, but if you try it out, let me know how it goes.