Shaper healing himself?

I tried killing Shaper and got his hit points down to about 2/3, then I watched his health rapidly climb back up to 100%. I noticed he doesn't heal himself all the time, just sometimes. Is it the Maven healing him? How are you supposed to beat him if he is constantly healing himself? I've beat Shaper three times before in other leagues, never had this problem. Can someone who's beat Shaper this league explain what's going on?

Yep, Maven can heal bosses if that's one of the boosts she takes to the fight. Take Frost Bomb to help counter Maven's heals.

I see, is there any way to tell when she brings that booster to the fight other than just watching the boss heal? Sometimes you don't realize it for a while if the boss is healing himself.

Pay attention to what she says "Take vitality boost" or close the beacon to your map device before open the fight.

It's not the boss healing itself--it's Maven who does it. She announces each ability she's using when she uses it. Just listen for her shouting about vitality, or keep an eye on the chat box if you need to, and drop Frost Bomb on the boss as quickly as possible.

There's also no way to know beforehand what skills she has available. She has random ones for each encounter she witnesses. There aren't many though, so healing is pretty common.

Fights like Shaper are ones where you already have to keep pretty aware of what is going on. Maven just adds another layer of that.