Tough boss because there’s actually two of them and many of the attacks can knock you off the bridge where you die and team mates can’t heal you. Weak point is the centre of their faces. Loads of ammo spawns in the centre of the bridge so remember to grab some as you run by.
As you enter the boss room only one will be visible on the left. The second one will be inside a cocoon high on the right. You can wait for it to spawn but, if you damage the cocoon first, you can force it out and on lower health. Then it will back off to charge up the beam attack and you can focus on the one already out. Would recommend someone with long range keeping an eye out for them coming back and shooting them in between.
When they are close to the bridge they do a regular damaging line slash and cone slash. Timing the dodge roll is essential because they hit hard. They also throw bombs you need to dodge that leave clouds of corruption behind. You can buy green leaf to neutralise this from the butterfly queen.
The closest one will then summon a load of homing orbs that also hurt a lot. Dodging is tough and you can destroy the orbs with one shot from a side arm. Distraction mods don’t seem to work on these either. We found it easiest to just shoot the orbs.
When this happens it indicates the other boss will be coming back with the beam attack so hopefully someone will have kept an eye and can warn the others. The beam does big damage and knocks you back so can push you off the bridge. It fires 4 beams in a row going from their left to right (or your right to left if facing them). So wait for the beam to fire first and then run into the area it just hit to avoid it. The other one will still be attacking so still watch out for that scythe and those bombs.
Afterwards they both start charging a scream attack. The only way to avoid this is to hit them hard and stun them out of it. They also become vulnerable in this state so it’s the best time to do big damage. The screams can kill everyone if both attacks hit so focus fire on one and then the other just in case you can’t get them both down.
After this they switch sides so the one you were close to will back off to charge his beam. The pattern repeats like this until one dies. Whichever one is left will be able to fire the beam attack now and this appears to be random instead of the previous predictable pattern. Hopefully the remaining boss will have low health enough that you can make quick work of them though.
There is a separate trophy for giving the heart from this beast to the Undying King by the way so remember that afterwards.
Good luck.