you can also combine magic resistance with lower resistance, to completely remove his resistance to spells before whacking him, after that just hit him hard with damageing spells and insect plagues, he can cast heal, and dragons cannot be interrupted when casting, unless it's because of a spell that disables casting like insect swarm, or plague, poison (if you've used doom/greater malison can be effective too(arrows of biting, the spell poison) you can also try having Anomen cast Harm, if he's lvl 11 (it will reduce him to 1d4 hp's)
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*peek-a-boo*
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