Poison clouds are the main damage but with mythic skills, more enemies can be affected as well as free dmg boost (for eg: 350% mh dmg from cosmic orb). That dmg boost contributes to the multiplying of the poison cloud along with the high regen, plagued and druidic.
Also by cronos' intention, more enemies affected, more damage and dmg multiplication from poison cloud because of plagued, druidic and good radius attacks. Mythics can definitely help. Sanctuary to make sure you're not OHKO'ed (one hit killed).