Occult Slayer is an ok class, but not great, and IMO it's unlikely to "mesh" with whatever build you're working on before entering it. You're probably better off finding another route for mind affecting immunity, unless you want to be a dedicated mage slayer (ie, with the feat). Then I'd say a Hexblade 3 / Monk or Rogue 2 (I like monk for all good saves, but either gives Evasion to go along with Mettle) would be a good set up to go into Occult Slayer. Either be a human and take Able Learner feat to make cross class skills less expensive, or dip Factotum (from Dungeonscape), which gets all skills as class skills. You could even dip Factotum AND take able learner, and then you'd basically treat all skills as class skills all the time...
Again, wht kind of role do YOU want to play? The build I described above, with proper feats (Mage Slayer, Combat Reflexes, Stand Still, Exotic Weapon Proficiency: Spiked Chain, etc...) would be a good melee character who can absolutely destroy any spellcaster he gets near. Obviously if you wanted to make a caster, that build's no good, though.
One of my old characters was a Dwarf Cleric / Church Inquisitor. Between wis-based casting and two classes with good will saves (not to mention the dwarven +2...), his will save was practically unbeatable without even any twinking. Lots of options you could go with...
EDIT: Yeah, Native Outsider would make it a little tougher for him to dominate you. If you do this, make absolutely sure not to brag about it to him at all. Try and let the fact you're not a humanoid slip under the radar as much as possible, don't bring it up at all. Then when he does decide to turn on you and doesn't use enough PP (be clear what he's augmenting it for, such as "You spending X PP to make it last day/level?"), declare it fails and he can't change how he augmented it after the fact. You now have an extra turn to revenge the hell out of him.
