I finished it in the Edron Orc Cave, while doing Orc Spearman as well, during fast respawn it is a good place, but you are right, lacks a place with high Orc density.
My recommendation is doing it with other bestiaries, in multiple places and not focusing just on Orcs.
There are some in Ab elvenbane, which you could kill while doing minotaur guards, orc warriors, etc...
Some around Orc Fortress that you could do while killing Pigs.
Some in zao, while killing sandcrawlers, and the mosquitos (forgot creature name)