Ruiner

I was surprised by this fight tbh.
As last boss I found this quite easy,even on hard.

Just stay behind the pod with your brother every time they launch their beam attack.
Pick out hosts comming left and right while they beam.

When you see a pause/cooldown in the beam attack.

Take a step left/right and shoot them before getting back behind your brothers pod in protection for next incomming beam.
You will see when it charges.

On hard I used grenades to help fend off hosts comming from both sides.