ok, you guys did hear about spells like Spellstrike, Breach, Lower Resistance, Summon Planetar, Summon Deva, Simulacrum, Abi-Dalzim's Horror Wilting... etc?
or perhaps Chain Contingency or Spell Trigger?
if you did, then what exactly is the problem?
now, you do have a mage in your company, don't you? at least one, preferably two (or three

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personally, I found the most effective means of taking out the main-baddies is to simply overwhelm them with as many creatures as you can get. for this purpose, I found the following items to be very useful - Vhailor's Helm (allows one of your fighters to cast Simulacrum), Iron Horn of Valhalla (equip it to the same fighter that has Vhailor's Helm and you can summon two Berserker Warriors - they are weak, but it's the numbers that count) and Golem Manual (again, equip it to someone that casts Simulacrum - two golems under your control). have all your mages cast Simulacrum, also summon a deva/planetar and any Elementals you can get (Elemental Prince's come to mind

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with such a force under your control, noone can give up a decent fight. for instance, in the FINAL final battle, I had two Iron Golems, two Berserker Warriors, an Earth Elemental Prince, a Planetar, two Keldorns, two Nalias, two Imoens and two of my main characters (dual-class fighter/mage). it lasted about ten seconds... maybe.
now this time around, I haven't gotten to Draconis yet (taken care of Yaga-Shura, now going through Watcher's Keep), but I plan on using some Chain Contingency/Spell Trigger combos... what I've planned out goes like this: Nalia and Imoen both prepare Chain Contingency and Spell Trigger in advance. with some careful management, the dragon should get hit with 3xLower Resistance, 3xAbi-Dalzim's Horrid Wilting and 2x3xMagic Missile (another alternative, somewhat harder to pull of, would be 3xLower Resistance, 3xMagic Missile and 2x3xAbi-Dalzim's Horrid Wilting) before he even realizes what's going on. that's about 300-400 damage (600+ for the second version)... InstaGib

or maybe I'll load the Chain Contingency up with Greater Malison spells and try the Quivering Palm thingie on him