The Demonic Sorcerer is encountered at the top of Dragon Keep. This is his third and final form.
The Demonic Sorcerer can summon up a trio of Nursling Red Dragons. These tend to get caught in particular locations. If they are not moving try to stay close to them in case you need a Second Wind.
He will launch Fireballs at you. He can also cast a Beam of Fire at you.
He can also do repeated Flame Burst by raising up then slamming into the ground.
The Demonic Sorcerer tends to fly for the whole of the first half of the battle with him. Once he does land though, he stays on the ground.
When you have reduced him down by one third of his total health, he will summon a Great Red Dragon. He can only do this once.
He can, and regularly does, make a shield around himself. Just blast that thing away with Shock-Type damage. He now resist Fire-type damage.
His critical point, needless to say, is his head.