Here's the cheesy way I managed to kill Draconis, but you need a high level thief (though not for easy way of killing human form.)
1. Reload from where you haven't talked to D. yet.
2. Bring your thief up to just out of D.'s field of vision. This is about at the top of the large circle on the ground.
3. Set the maximum number of traps possible in the area, resting between if needed.
4. Bring thief back near entrance with rest of party.
5. Bring mage/cleric up just out of view. Cast area effect spells slightly back from where D. is standing (still not hostile). It's possible to do this without being seen. You may want to cast farsight to see where he is.
6. Move mage and cleric back to safety.
7. D. will just stand there and take damage. Magic resistance will negate some of the effects, but if you cast enough spells he will eventually die.
8. You've now defeated mage form without taking any damage. If you set enough traps, he should die almost immediately after turning into a dragon. If not, you have the choice to close in to finish him off, or leave the area and regroup.