The Shield Maze is impossible

I play on difficulty Hard and the Shield Maze is clearly impossible, I'm stuck at lvl 2 for my party while I'm encountering Water Elemental (lvl 8) and those f*cking Corrupted Mongrel Brutes (lvl 5), one alone is hard enough because it basically 2HKO any of my character and OHKO my rogue but I can manage it, but he's not alone, he has a full party with also overleveled Sharpshooter. I think I've got rid of every enemy at the upper level but here they are clearly overleveled.

Any tips ?