Yeah, I'm thinking that his mistake is no mistake as well
As to how I'd handle him, especially since he's a 'mistake' and inherited from another game we ll, I think I'd probably be an ass about it. This character is bound to have ticked off someone in the past, either someone with power or who has access to someone with power. I'd find the nastiest critter in the various books (hey, he uses all these third-party books without, it sounds, explicit permission to do so, so sauce for the goose is sauce for the gander) specifically to take him out. It's been sent by Villian X to deal with him once and for all.
Without looking all over Creation, the first thing that springs to mind is Rakshasas with Monk and Rogue or Assassin abilities. The spell resistance alone should give him pause but you know, If you haven't used them before I'd say to heck with it and use the 3.0 version, where it's
invulnerable to
any spell under 9th level. His spells and probably most of his fancy weird abilities are then useless; the highest Veil possible for him is a 7th level spell equivilant so none of them will affect the creature. Honestly, I usualy bumped their CR a few levels and made it so they were not
affected by magic of less than 9th level. You got bracers of defence? Was that made with a 9th level spell? Why, no? Then they're useless against it. Four or five Monk levels, dex, feats; the first one
will win initiative and then Initiate boy can stay Stunned each round with his wonderful primary spellcaster Fortitude saves while the second one uses Sneak Attacks on him. I predict two rounds later, the Initiate thuds to the ground.