Try having your mages cast lower resistance, then blast him with tons of magic missiles (spell sequencers or spell trigger works nicely) and have your fighters hack away. Summon insects and elementals from clerics and druids can also help greatly.
Or you can cast wizard eye, then send in couple of waves of summon creature to force attack him (have your own party members remain out of sight) and let the dragon wastes his more powerful spells. Then charge in and do the above.
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