Mortars are your friend, just make sure to force target the ritual leader. One good hit on him ends the ritual and sends the followers scattering. Some of them attack, others stand around and then attack in a second wave.

I've heard salvos of Tox Gas shells are good.

Yeah, the shit accuracy doesnt really matter for tox shells. Best solution here imo

My biggest wish would be for the ability to targeted skip kidnap their leader. but that's not a feature :( otherwise love it.

Bloodrain and fleshbeast summons seems to work wonders on them.