I got defeated by Metatron and will have to restart once more, I think just a couple more fights before the last one should be enough, I did almost none at that point.
The game is atmospheric due to its awesome gfx and music and the inspiration behind the game fits the theme perfectly. For sure there's little you can do except for choosing characters who won't randomly die to one hit then grind a bit and go for it, but given the context and the fitting game length it's fine.