At the beginning of the mission, there are a couple dozen Mursaat. Wait for Brechnar Ironhammer to do a Kilroy Stonekin, making it easier pull smaller groups.
This mission and the following one, Abaddon's Mouth, both have significant numbers of mursaat throughout, so simply running the missions should generate a decent supply of trophies.
You can find groups of 4-7 Mursaat wandering around throughout this area. Just outside Thunderhead Keep there are at least 10 closely packed groups that can be farmed quickly. If you venture further into the area, beware the groups of Ice Imps (unless you are prepared for their Maelstrom).
