I got the basic idea from other MMO i am playing.
Boss has a shield that makes it immune to damage. If taunt debuff is applied, shield is broken, but will re-apply itself if taunt debuff is gone.
If player was not healed by group healing skill (skills that heal others, self heals dont count) for X seconds - they begin to lose health every second until they are healed by a group heal (they can use group heal on themselves).
This way someone has to slot taunt, someone has to slot group heal.
PC EU: Senilia Bright - Healing Bard. I live in Kiev, Ukraine.