30 hours in and still get bodied by Tricephalos every time

Title.

Preface: I can play solo and get to Tricephalos no problem.

When playing with a group (pug) we're hitting bosses and greater bosses, gaols when possible, I'm prioritising seals, collecting talismans, grabbing flasks -- effectively gearing up as best I can. Usually hit level 12-13.

Whenever we get to day 3, we just get fucking wiped. It's barely even close.

I'm starting to get pretty frustrated. What am I missing?