This boss battle is painfully slow.
It is a train with a huge "health" bar with repeated cycle.
It comes with some minor randomness with portals that tries to cross over you, some scenario patterns and the important one: when the tracks are drew just above the height of your head. You have to shoot below it to get criticals.
Sadly this takes a lot of time so the better is to have a repeater weapon with a huge magazine so you won't have to wait for reloading time. I had the chance to use a shotgun with my FL4K skills when I did critical it gained one bullet, so it never ended emptying the magazine.
But anyway, it was a long, long battle, 15-20 minutes of 2 people shooting at it meanwhile surviving the lunatics that leave the train.
It is not hard, just time consuming.