Haven't done these fights yet, but here are some thoughts I have.
critical damage thief build
his damage output is insane
You're a thief built for DPS so it's likely your defenses and resistances are low which is why his damage output feels that bad.
Switch off melee and try using thrown/ranged weapons or switch to spells.
can deal a ton of crippling debuffs
If they're that bad, use the stance power Equilibrium w/ the Silver Mask to be immune to them.
grenades barely deal damage to him
Ley grenades have always stunk imo, but if you're using items don't forget you can prebuff with certain ones. Not sure if you can't for that fight though.
he shrugs off Abjure like nothing with 3 action turns per round
If you're going Abjure with Twist the Knife, you're only getting 2 full rounds of uninterrupted DPS. Try switching to Charmer for Expert Inspiration and use Unbreakable instead. Doing this will net you 6 full rounds of uninterrupted DPS and also give you time to set up buffs to do more dps like Song of Storms, Blessing, Charge Weapon, etc. Of course, this is only uninterruptible if he isn't constantly disabling you with disarm or stuns.
I think a dual wield spellblade Charmer with a kunai (for the bleed) and Unbreakable should be able to out DPS his regen and be able to knock him out within 6 turns. But I feel that something like this also would delete anything.