Ravager - bane of mages
I take it you are going with a very magic heavy party, as I did (three mages and two clerics), in which case the ravager is seriously tough.
I don't know how you managed to lower his resistence (did you see it come up or did you just cast the spell) since I think he is resistant to all spells below a certain level (definitely 6, perhaps 8).
Add to that the fact he will make his saves (and you have no way of lowering them) makes him fairly immune to magic.
What I would do is change your high level spell to summons and then buff up your warriors. Remember that a wizard (with staff of ram for instance), buffed up (stoneskin, mirror image, protection from magic weapons) and with tensers transformation is quite formidable.
Devas, planatars and elemental princes should hurt him or at least keep him occupied. I think he regenerates very quickly so just concentrate upon him and forget his allies.
As userunfriendly said you need +4 weapons to hurt him so dig them out or go and buy some.
I think spellstrike removes all protections, but I will have to check.
If all else fails download the acension mod, then your main character can turn into the ravager and take him on mano a mano.
Personally I am not looking forward to facing him with my solo cleric/mage
