It’s not a boss.
It’s the Witch from Left 4 Dead.
You’re not meant to fight it. You’re meant to avoid it. The fact it attacks one person relentlessly, and kills them with no hope of rescue, is exactly how the Witch behaves.
You can kill them but it doesn’t matter. We ran into three in one mission. The first time it killed two people and left. The 2nd time it killed two people and the remaining 2 killed it, getting us a penance. The last time we just avoided it.
You’re just meant to avoid it. It’s a stage hazard. Or it’s there to ruin your runs when it’s in the middle of a horde you grenade.
That said if you don’t shoot it, you can be within Zealot dash range and it won’t aggro. Doesn’t care about noise or activity near it, it just cares if you attack or get too close.