The last boss has 3 phases. In the last phase the boss has 999 HP and it summons all kinds of Spectres.
In my case the boss summoned a Spectre which I could only hit once because after the first Point Blank Shot it moved to the last rank and I could not damage it with any of my characters. This Spectre was doing 50+ hits per round and it simply dissapeared after 20 rounds (glitch!?). But at that point it already killed two of my characters and there was no point in continuing.
Basically the last fight takes like 20-30 minutes and only specific party(s) can make it till the end. Absolute garbage design.
I guess I got what I deserve for playing this piece of shit.