With that make up you could run a skill challenge of sorts. The party would have to perform a series of skill checks of varying difficulties to perform a ritual to try and cleanse the cloud. Something like inscribing a runic circle, trapping it inside it, then channeling energy to dispel it. If the cloud has some elemental minions with it, it can make for a tense encounter.
My suggestion would be to allow the druid to research or know of a ritual to help.
Step 1 is to inscribe a runic circle 3 successes before two failures (Arcana DC 17/Nature DC 15/Religion DC 20) add two to the DC each time a given skill is used(So three nature checks are DC 15, DC 17 DC 19) Failure summons an earth elemental or something less nasty while also finishing the circle.
Step 2 would be to trap the cloud in the circle. It can either chase the players or they can lure it some how.
Step 3 is: once in the circle they can spend spell slots to attempt to dispel it (Casting Ability check + Spell Slot Level Burned, DC 16). If they do this and there are Earth Mephits or something harassing them it will force them to make choices. I can imagine the barbarian holding off an earth elemental while the druid is fumbling with some chalk and herbs trying to make the circle.
All numbers are subject to change, these are just rough tier 2 numbers off the top of my head. The skill split allows each character to have a reasonable chance of helping make the circle if they want to try and rush it. Otherwise I think with a few summons you have a cool brew going.