[Thor]

Looks to me that they have to be at full health and drain you of at least 80% of your life to give you a point and it seems to give you 1 more point for each 5% more of life they hit you for.

These may not be fully accurate but are close to what I experienced so far. I got various points from Mr.F, between 1 and 4 so far. (he seems to be the only 1 that give me points, others are way too strong or way too weak...)