The black pearl- james pidgely- Defeating Claude d'Arcis
 

So I know where the den of thieves is and I've been trying to kill all the different cadets and thugs there, but I still haven't gotten Claude d'Arcis? What am I doing wrong?

IIRC, I didn't get him at the Thieve's Den fort.

Instead, I went through the Tortuga Graveyard (entrance past Fabiola) and down the hill to the right. When you enter the cave there, you come across a little military encampment. Claude d'Arcis was disguised as one of the EITC soldiers there.

Good luck!

Yep, that's what I did too... and you've just got to keep at it and you ought to get it after a few more tries.