[Sharpedge]

Welcome to Ironworks Graywolf.

Most standard practices for beefing up your party apply to this fight. This includes casting Haste, Chant, Defensive Harmony, True Sight etc. It helps if you have these spells stored in spell triggers so that they are cast instantly.

Distract Gromnir with a low AC high HP character and have your other combat characters engage the mages as soon as you can. You may be able to kill them before they get their various magical protections up. If not, be sure to have some spells like breach and ruby ray of reversal handy.

The other NPC's that attack will most likely target your mages, so make sure they are protected with fireshields and stoneskins. Again, spell triggers help. A reasonably competent combat character or a good missile weapon should keep these NPC's busy until you can bring your full force to bear on them.

Once the mages are dealt with, hit Gromnir with everything you've got. He should go down pretty quickly. Once he's gone, mop up the rest of the NPC's.

Keep your belts well stocked with potions, and don't forget to use your new high-level abilities like whirlwind attack.

I haven't done this fight in a while so I've probably missed some crucial detail...

NB: If you give more specific details on your party composition then we can probably come up with some more directly helpful tactics.