Stuck on Acheron Archdevil boss

[SSSIIIIIIIIIIIIIIINNDDWWRRYYYYY]

After many tries I know the level very well and when to unleash my weapons' ultimates.
The wall I'm hitting since months now is the boss of the level.
The boss itself isn't a menace. It's slow and projectiles are easy to dodge.
The big problem I'm facing are the many mobs it spawns throughout the fight.
I can take out wave 1 and two easily by sucking everybody in the crossbow's black hole and DPSing with dogs' doppleganger. But when that archangel spawns it's game over.
For some reason, once the archangel spawns, the health crystals I've kept intact don't give me any health when broken and the angel manages to hit me with its beams even tho I dodged and I'm located at roughtly 5 meters sideways from them.

This kind of... "bugs"? only happen during this fight against this specific archangel. The 3 others in previous arenas don't do that.
Is this archangel a mini boss? A super version that isn't mentioned? I lack 2 bestiary entries in my codex so I wondered if it's either one of these 2 or just a bugged mob for extra sadistic difficulty. If it's the latter, how am I supposed to beat a health regen canceling, aimboting sentry?