My best suggestion: Make the DRK unable to go passed 1HP for 10sec, but also lose 50% of their current HP every global tick.
By doing this, it benefits healers to delay heals since the DRK will lose more HP the more healed they are.
Keeps a lot of the "challenge" but doesn't f***ing kill the DRK at the end.
Edit - Furthermore, it doesn't completely gimp LD if a healer uses Benediction immediately after.