I'm perhaps a little late to this party, but I just killed the Adra dragon on my first attempt, without even planning to fight it. I had heard a lot about how hard it was to kill and didn't feel up to it yet. My party is level 12 and consists of my protagonist (a barbarian), Eder, Pallegenia, Durance, Grieving Mother and Hiravias.
I intended to talk to the dragon to get its quest but I messed up and it went hostile. I thought 'oh what the hell, I'll give it a whirl before reloading'. The one precautionary measure I had taken was camping to get the beast accuracy bonus, but that was it.
The first thing I did was have Hiravias cast Hold Beast. It worked! But then I realised that no one could hurt the damn thing. It seemed immune to almost everything. So I kept it stunlocked with both Hiravias and Mother's Mental Binding while I tried to figure out what would work and keep its xaurip creeps at bay. Once I realised only crush damage seemed to land I used spells like Twin Stones and Ectopsychic Echo, along with my barb's Dragon's Leap.
All of a sudden the Adra Dragon went from Barely Injured - which it had been for about 10 minutes - to Near Death. Then BOOM!! It was out.
Sorry if someone has already covered this method, I didn't have time to read the whole thread.
