The trolls in question are in Talus Chute, in a cave due west once you've exited Droknar's Forge.
As for HOW to troll farm, don't ask me. I only know where they live because I got a friend who farms them. (He once took me along to watch - it was something, seeing that many trolls converge on a single warrior and losing.) I, personally, have always made my money the old-fashioned way: Kill everything I come across, Loot everything they drop, and Sell it all to the merchant.
Slower than these dedicated farming techniques, but in my opinion, less boring than fighting the same foes over and over. The money does add up over time, AND you can pursue other goals while you do it.
"As the good archmage often admonishes me, I ought not to let my mind wander, as it's too small to go off by itself." -Danilo Thann