generally speaking when I do this task I stay to the right along the wall, which limits the amount of mobs that can wander over you, and allows you to cherry pick the ones you want to kill... like the ones that are NOT enchanter goblins.
I also set up GINA triggers ( easy enough to do ingame audio or chatevents) to warn about the catapult, you get an e-mote 10 seconds before the hit. If you time your pulling right, you can move, engage and kill... before the next catapult comes (every 60 seconds) provided you have toons having issues with it.
If your crew is not overly hampered by the catapult, then obviously just bulling trough works... but I still keep to the right wall and look for piggies and minos. By the time you start running out of them you ought to be around near the named dudes and enraged =)