The PHs for Eldrig move around the whole zone. The best way to do it is to have toons stationed at the spawn points, and have your bard Highsun Song all the mobs in the zone back to their spawn point. If any appear at the Eldrig spawn points, kill them. After killing enough of the PHs you will get Eldrig. It's a pain. If you are able to hold out for the next increased rare spawn bonus, that makes it much easier. Or if you have enough to use the Agent of Change, he might be active in that on the first go.