Either. I would gear him out in, obviously, defense/shield and absorb first and foremost.
You can use a higher endurance rating, your fight will just take longer. If you feel comfortable with your own healing skills you could sacrifice end for might armorings and mods so he does a bit more damage. That way the fights won't last as long. Plus you don't want him 'too weak' to the point when you throw out a couple of attacks you grab all the aggro instantly.
Heal spec have no issue with threat on the target khem is focusing, unless you REALLY let his gear go out of date compared to yours.
I leveled until 44 as corruption, and ashara could keep threat on a strong or elite against the meager dps I was throwing out in between heals.
Even my well-geared dps sorc have a certain delay before pulling from xalek, who has tionese weapons and columni mods (versus full BH mods, DG armoring hand hilt).