M1S - Raining Cats

Assuming the mistakes so far are minimal, the chance is pretty high that the boss is defeated before you get to see this mechanic.

The boss begins to cast Raining Cats, which does two things. First, it will swipe the players who has the tethers with a protean, inflicting a long lasting slashing debuff while doing so. Second, it will attack the nearest and the farthest player from the boss with a stack. This mechanic looks daunting if done the intended way, but luckily, this can be cheesed very easily by following this strategy from inumaru:

MT and ST should immediately grab the tether and stand max melee from the boss like this. H1/D1/D2 should stand inside the boss's hitbox at south while H2/D3/D4 stands on the waymark C. During this mechanic, the ranged players should not be moving once they are in position. The stack AoEs are quite large, so make absolutely sure that the near group stays inside the hitbox of the boss to avoid accidentally being clipped by both stacks.
The boss will launch her proteans and stacks every time she finishes casting Raining Cats. The tanks should take the first one normally, and use their invulnerability for the 2nd and 3rd one. The healers should heal when necessary.
After the 3rd cast of Raining Cats goes off, the D1/D2 should swap place with the MT/ST to soak the final Raining Cats. Congratulations, you just cheesed this mechanic.

Predaceous Pounce

If the boss is still somehow undefeated at this point, then the boss will summon a clone which will execute the Predaceous Pounce against the party. This is the same mechanic as seen in normal mode, with the added twist of also seeing the One-two Paw while the clone is jumping around the arena. Locate a safe spot around the middle of the arena, and dodge the incoming attacks as necessary. The One-two Paw is not lethal and does not inflict a damage down either, so feel free to heavily mitigate this and/or heal through it.

Enrage

If the boss still somehow is not defeated at this point, then the boss will now cast Mouser for the third time. If you allow the boss to finish her cast, then she will proceed to demolish all the tiles in the arena, effectively throwing the whole party out of the arena.