The only thing I've found in the game prior to this that is as cheap and hard as being a new character and running into the hp regen nurgel banner bearer is the time I got overwhelmed by a nurgle dreadnaught with grenade launcher chaos marines for backup. Grenade in the face, knockdown, claw to the head. 2 seconds to stand up. Half way there... Grenade in the face, knockdown, claw to the head. Repeat until you finally get knocked so far back they can't reach you in time or death.

He was really tough doing assault/melee as a starter mission - had to kite everything out of the room, then rush him and spam the heck out of attacks till I killed him, and then run like hell from things I didn't kite. Unfun. Thankfully the handy mission rating tool let me indicate that!

He was tricky, but even at level 1 I managed to take him down. Running a Sentinel, turned out the secret was to just try and kite the Nurglings long enough until I could drop multiple turrets, and that eventually upped my DPS enough to overwhelm him and his healing. Otherwise with a heavy bolter you're just gonna have to soak some damage for a bit while you primary fire spam him down a bit.

Hammer/shield combo seems to work well for me too. Anything with reliable knockdown, really.

It does seem like his regen stops at certain points, but those points aren't really made clear. My current theory is it will stop for a while if he has no banners and he has just summoned the Nurgling swarm. 

But agreed, the Banner Bearer was an unpleasant difficulty curve going from "satisfying murder stroll through a field of unarmed Eldar" to "trying to repel a Black Crusade with a broken stick" 

Before I found better weapons 3 turrets helped to beat him. 

You can Interrupt is healing by knocking him down. He is really easy to beat with the 4th skill of a heavy bolter because he is always on the gorund with it.

Craft a heavy bolter if you have problems to beat him.

But I agree his healing a bit to much. Really hard to out dps is healing without interrupting him. With the wrong weapon it is nearly impossible to beat him.