It appears that if there are "friendlies" on the battlefield who are flagged as non-combatants, they will take continuous damage from ground effects completely outside of the combat rounds.

In the "murder mystery" final battle, despite my casting rain on the battlefield, one of the enemy mages cast a fireball which included Zombie Jake in it's area. Zombie Jake managed to die from burning damage before the rain doused the fires on the ground. It appears that he can be killed by creating any burning ground, simply light the ground on fire, and wait- he'll keep taking damage without turns ever changing.