Just remember, this method works ideally when you gather a group of mobs that are purely melee-class. You don't want to be pulling any ranged classes or casters. Sometimes there are mobs that charge after aggroing, which completely disrupts the pulling process. So skip these. Here are a few of my choice favs.
68-70
Wooly Rhinos, Borean Tundra
They travel in packs, and are often accompanied by bull rhinos. Just stay away from D.E.H.T.A while you're doing this, k?
70-71
Gorloc Waddlers and Gorloc Steam Belchers, Borean Tundra
These guys are scattered close to the coast, just south of the geyser fields. You can probably grab a good number of them and then lure them onto the main path, ensuring that you don't aggro anything else. Just start at one edge of the stretch and work your way down to the other edge. When you get back to the first part, there will be respawns. Excellent group.
Iron Dwarfs, Howling Fjord
There's a large line of them surrounding the excavation. You can pull a few of them, but don't do too many, since their attacks leave a stackable DoT that is sure to kill you. A caster patrols the area, but you can pull it separately from the pack without too much trouble. Respawn is fairly good.
72-73
Wastes Diggers, Dragonblight
These guys have stackable DoT's as well, so watch out and don't pull too many at once. You can rotate between excavation pits and they should respawn by the time you get back to the first one. The Taskmaster is ranged, so be sure to pull him seperately when he patrols to the edge of the area.
Scarlet Onslaught, Dragonblight
Northeast of Wintergarde, there's a large encampment of Onslaught mobs that are purely melee. there are so many groups here that you can have a field day with multiple pulls and respawns will occur when you go back to the beginning.
74-75
Highland Mustang, Grizzly Hills
OH MAN. There are a few herds that gallop in groups of 8-10. These are the ones you want to go for. OH BOY ARE THEY GREAT TO GRIND ON. Remember. Arcane explosion to have the circle collapse on you, then Frost Nova in place. Highly Recommended!
75-76
Venture Corp., Sholazar Basin
JACKPOT! The entire Venture Corp Encampment on the northwest side of the Suntouched Pillar is made of melee mobs. Plan out your pulls, and be sure to stay a safe distance from the "boss" mobs. They're not elite, but they have some nifty ablities that can throw off your pull by a lot. As you work your way through the encampment in a circuit, you'll hit respawns again. As a tailor, I get about 3 or 4 stacks of Frostweave in 10 minutes of AoE'ing here. Great experience as well.
Will post the other levels when either I find them in game or online.