What makes a boss reset its health?

[Inciteful]

Do you mind sharing this strategy to us, which maps, which boss? PM will be good as well,hehe thanks.

Soloing tombs comes to mind. You can't focus on the boss (or beat him, in some cases) with 10+ adds or so. So... pull the room and kite/clear.

Also works for groups just strong enough to take down the boss (by itself).