One of my favorite grappling builds:
Race doen't matter. Skarn work well just for more damage, though.
Sorcerer 1/Cloistered Cleric 3/Geomancer 7/Barbarian 1/Black Blood Cultist 8 Trade Knowledge Domain for the devotion and take Planning. Other domain doesn't matter much.
Feats:
Percocious Apprentice
Track
Improved Unarmed Strike
Persist Spell
DMM (Persist)
Extra Rage
Power Attack
For Drift, you choose the Rake and Gore at 3 and Improved Grab at 4. All Ley Lines go to Forest terrain for Acorn abuse (probably cast by a party member, but a wand works too).
Persisted Divine Power is a must, with Divine Might to go when you need an extra umph.
When raging, you have a couple of options. Attack with your claws and bite normally. If any of them hit, free grapple check. If you succeed, you get to attack with claw/claw/rend/bite/rake/rake/gore/arm spine/arm spine. Except, of course, these are all auto-hit. On their turn, they make a check to escape. If they fail, they take claw/claw/rend/bite/rake/rake/gore/arm spine/arm spine. Then, on your next turn, you keep attacking the, making grapple checks to do so. Each one you succeed on is automatic claw/claw/rend/bite/rake/rake/gore/arm spine/arm spine.
Oh, and you still have 5th level Cleric spells to fall back on should you need them.
Abilities are Str->Con->Wis-> everything else. Str and Wis can be swapped if necessary, since the former is much easier to boost. Cha should be at least 12 base