Look to the far northeast of the Enchanted Sea area, in a small lagoon. There's only one centaur there, Gineleng. That's who you need to slay. Beware the "bugs that are driving him mad," though! They're Mytharian Wisps and they can literally cause you to go insane. They lurk just behind him.
Also: Yes, taking your Monk to a Warlock would be a good idea...interestingly enough, Warlocks can train in Kung Fu as well, so you can continue training him at the Bushi Dojo guild.
-Sazerac