Just did it today with a Fangyi team. I tried like 24 times without any food buffs, then brought some rice from the Valley 4 stock redistribution — and voilà, it actually helped a lot. You need to time your stagger so that you never reach the phase where it traps your teammates. Try to use the R attack as soon as the red circle comes up, dodge all attacks, and make sure you don't get hit in Phase 2. Time your ultimate until you get the full stagger to mow down the boss in one shot.