good thing to remember is, that once his 3 guards are dead, he will perform resurrection for some 5 seconds - good for dynamite and shooting onlookers
did it on 4th difficulty
at start, when he is slow, soften him up with a dynamite, which should kill his followers and force him into resurrecting stance - giving you 5 seconds for another well placed dynamite
then play with whatever gun you want, keep some 10m distance and no souls should spawn. Venom, panzerfaust, shotgun, MG42 and somewhat teslagun are useful against him
Venom is the fastest, giving you the least opportunity to take damage
once onlookers show up, kill his guards with flamethrower to force him into resurrecting, giving you time to kill onlookers with Snooper rifle