I have spoken with {Ashley} and based only on what it has done so far, you can assume that it is vulnerable to magic and blunt attacks, it is immune to piercing, it's an acid type, which means it might not be immune to poison. It also has three actions per round so though it's slow, it can combine moves. It's also a bullet sponge and it's immune to extra damage from critical hits.
Cat can punch with claws retracted or pick up rocks rocks to attack directly but damage will be limited to improvised weapons. However, nick and dual wield will still be applicable, so you, {Ashley} and Alice can still punch it to death, but ending a turn within 30 ft of it may lead to multiple attacks on you and it hits hard and may grab you, contact will cause acid damage as it digests you.
I recommend rage, give your temporary hitpoints to someone, {Ashley} won't put herself in a position to be hit again. Cat will be susceptible to the acid damage but will take half damage from its pseudopod attack.
