In the desert of Southern Ro, look at your map. The North West Shore of the lake where the Spectre Tower is located is where you want to be. There's 11 Orcs there, some are on the shoreline and some are just a few feet up the hill by the palm trees. Their respawn time is roughly 20 seconds. I already posted this for the Achievement only requiring 100 Orcs. It will work perfectly for this achievement as well. I parsed the kill ratio per minute here, and I ended up with 20 kills per minute, so figure roughly 50 minutes to complete this achievement if you started at 0 Orc Kills :)
Edited, Jan 15th 2015 5:14pm by VonsneerTheScoundrel