Successful persuasion leads to the NPC leaving but promising to return.
A group of "Skeletons" (Faer Gorta) gets spawned by Balthasar at the beginning of the fight. Once you kill a bunch of them Balthasar spawns another group. This ends as soon as you kill Balthasar.
As a mage simply use Mark of Flame a couple times and combine it with Chakrams that deal fire damage too. Balthasar moves within the groups of Faer Gorta, so simply aim at him to kill him and the Faer Gorta together. He falls sooner than you would actually think after what he had to say prior to the fight.
As a rogue or warrior, use fast or very fast weapons to have a better chance to interrupt any spell in course. Just focus Balthasar and probably you will have him killed even before the summons reach you.
Any answer other than a successful persuasion check will result in a fight.
He claims he will return, but never does.