Take Your Best Shot
There's another purple shrieker ahead of us. The giant mushroom gets its name from the shrill noise it emits when it detects movement nearby. I have no idea what evolutionary purpose this serves, but if it shrieks the Hellknights looking for us will have a good idea which direction we're moving in.
We can't move in to melee to cut down the shrieking mushroom, but most of us have ranged attacks that we can use. We're relying on the ranger, adept with the bow, but the wizard, bard, and paladin all step forwards to help. Coordinating their attack, on the count of three all four characters use their weapon of choice.
The bard's whip lashes forwards and strikes the shrieker!
The paladin throws a knife that sticks directly in to the purple mass!
The wizard's crossbow bolt thuds in to the mushroom!
The ranger doesn't know which end of an arrow to hold, and it clatters ineffectively down the sewer!
The purple shrieker we attacked stays quiet, its life essence drained by the three hits, but the second, unseen shrieker around the corner is startled by the arrow skidding past it and lets out a high-pitched noise, a bit like the squeel of our ranger when he fumbled his attack. Still, at least we have three competent members of the party when we need to rely on ranged attacks.