Two easy plans that I’m not sure if would work
1. Robot army attack on the hell demon, if she can’t eat souls she can’t heal. Elon must be powerful enough if he’s on the exorcist leader table.
2. Teleporter brings demons to fight, train against, and eat, and then just bring a lot of demons for him to eat during the final battle.
If I were the hell demon I’d get someone with mind reading/control, find out where the bases of the exorcists are then eat the weaker ones and work my way to the leaders.