Make the Obsidian Crown disable all healing

[Sundance]

Shoot him thrice with a laser gun.

He will not have time to heal and just vanish in a cloud of misery and pubbie tears. Lag will keep you from teleporting away until after they're gone.

That's if he's a pubbie though.
If he's anyway experienced he'll at least dodge two of those shots and you'll get teleported away, in which he'll immediately use some burn meds.

The thing with the obsidian crown is that it's a bit of a old relic of the past that falls into the artistic toolbox bracket where
A) It's a rampage item
B) The only hard counter against it is if the user is inexperienced.
If the user is anyway experienced, he will rampage. There needs to be a hard-counter that a player can use against the wearer, which is why I think chemicals should work, OR if you want to chose a less lethal method than pouring sarin on his smug face, knock-out chems or an anti-psychotic (another use for the tranq rifle) could disable the powers for a short while