Erik had nothing to prepare him for demons, however each of his other souls had slight advantages to speed their recovery.
John had seen all the horrors of war and participated, he was the last spartan with the Hyper-Lethal rating for a reason, and he was trained to keep cool.
Alric was a dwarven soul, naturally immune to the predations of the warp, resistant to magic and the powers of the mind.
Aleandrea used to deal with fey, demons and other things on a nigh daily basis, with a god-like patron fey in her head before her pact was resolved.
Eldrad was THE eldar psycher, trained well beyond and skilled well beyond even the most tightly bound psycher to resist mental influence and drive out demons.
And so, when the devil charged, controll was regained.
The staff couldn't moved far enough to impale the devil, so he let it fall a little moving his hands. The Devil struck him with force greater than a charging brute, and even with the magical enhancement of dwarven runes, his shields flared and almost died on impact with the devil and then the ground, they couldn't take much more. But to say Erik was reinforced physically was an understatement.
He was almost as tough as a tank.
And could flip heavy tanks two handed with ease.
His hands, spread a little, pushed the devil away but a little, enough to move his hands, then darting up in a blur of motion, he claps his hands around the devils head with all the force he could muster.