Depends. I don't really "grind" per se but I've had good luck AoEing the naga out in Zangar, the ghost alliance in Hellfire, bugs and rays in Zangar, the vultures in Hellfire were superb. Lots of stuff, really. This is where mages are awesome because if you set up counterspell on a round-robin macro, you can silence the casters during a round-up and get a nice pack of both. You'll have to find the spots that work well for you and your style. Things change with different spells or macros or setups.