Arelith

I just tried to fight Darlantir in the fire mines of the Burning Shores (the drow mage with scimitars) he felt like he needed tweaking, considering that he doesn't seem to be intended as a boss to a level 30 dungeon.

I was solo, so I can't be certain that it was simply a case of him being a hard counter to my character, but he seemed impossible to damage. With 42 AB I needed a 20 roll to damage him, on top of which he had regeneration and concealment. It simply seemed impossible to damage him quicker than he could heal. I tried using a gonne, but he also hits very hard (weapon master level damage).

Is Darlantir tuned appropriately for what he is intended as? He didn't guard any special treasures, just a couple of standard chests.

Yes because he himself has a valuable item on him that I won't say because I imagine DMs would edit this message to say FOIG.

He is meant to be tough because of the item he holds

He's an optional boss that is supposed to be hard. You said yourself you were soloing, and not everything is meant to be soloed, especially when it comes to epic content.