Getting knowledge from these took pretty long for me, and there are only few of these trolls around the Anti-Troll Fortification. You can speed up the knowledge learning by using a fluid collector on each Troll Siege Soldier you kill. (You can also learn the knowledge of a monster by gathering it.)

If you get that knowledge fast, consider you lucky ;p

I found a bunch of these guys surrounding the walls of the Anti-Troll Fortification along with Trolls, Troll Barricades and Troll Warriors.