[Barry the Sprout]

If you fancy Cheesing this one then a couple of dodgy tactics do the trick. Firstly you can try the old tried and tested Set Traps trick:

Set Traps in the archway that is the entrance to the ruined temple thing. Put you party behind them. Send in a Planetar, who will have no problem with Draconis's earlier form. Draconis then kills the Planetar as a dragon and stalks after your main character, in the process he walks over the traps. If you have enough of them this should do the trick.

Or you could try the Polymorph option:

Acquire a scroll of the Level 9 Polymorph spell, trun yourself into a Mind Flayer. As you kill Draconis through intelligence drains he won;t turn into a dragon - he will just die instantly. Also he will fight you as a human, in which form he cannot damage a mind flayer.

Both of those are incredibly cheesey though, not recommended for anyone with any amount of self-respect. Good if you are really bored of fighting him though.

Other than that I can only reiterate what the others have said - true Sight is incredibly helpful. Keep casting it over and over, use loads of Summons, Greater Whirlwinds, sequencers, etc. Just buff like crazy and throw absolutely everything at him. Its surprising how much of a difference a bit of preperation makes.