DM
There is a straightforward way to kill a single pair of purple worms. Retreat
to a 2×2 open space. When the worms face you, strike (a battle axe is nice),
then sidestep and turn to face the square you just left. They will hesitate a
tick, then charge into that square and turn to face you in your new square. By
this time, one of your fighters ought to have caught her breath and be ready to
slug them again. Repeat until sliced.
Note that the brainier worms won't always charge into the square you vacated,
but will sidestep themselves. Be alert for this and move accordingly.