Aridha didn't know what the thing was, but she knew trouble when she saw it. Though caution had prevailed before, she wasn't leaving the others down there to fight alone...and this thing was far too large for her magic to manhandle from a distance.
She went to the wall and vaulted over the side...transforming as she did into the enormous spidery monster who's form she'd taken once before. It easily clung to the wall of the chamber instead of plummeting below, and while she skittered forward, most of her attention was on another action entirely.
Her far back legs dipped up and made a complex movement, weaving silk from spinnerets into a globby, sticky wad. It was a simple matter then to fling that ball of webbing at the creature in hopes of slowing it down long enough to get down and help!
(Move to T14 and wildshape to spider, then finish move to T12 along wall and attack with Web. [roll0] to hit; on hit the target is Restrained (meaning it can't move, and attackers have advantage) until it succeeds a DC12 Str check to break free.)