Since Cataclysm has been released and the entire old World of Warcraft was reshaped for almost every zone from 1-60 a lot of the AoE Grinding Spots were nerfed or changed. It was Blizzards intention on removing almost all of forms of AoE leveling from the game because they wanted players to do quests and follow the storyline and lore instead.
However, there are still some very good spots to AoE level at for certain levels.
Westall Levels 13-14
The Dust Plains Gnolls. This area is still highly efficient place to AoE level because there are camps of gnolls. You can get 3-5 per pull and there are enough enemies that you can essentially keep leveling here without interruption.
Wetlands Level 22
In the Black Channel Marsh there are Mottled Screechers that are about 3-5 in each camp. This is a good area to farm leather in and there are enough raptors against the mountains to keep you going consistently in AoE leveling.
Arathi Highlands Level 26
This one is Horde only but one of the best places to AoE grind. They are humanoids and drop cloth but at Dabyrie’s Farmstead you can get 3-12 per pull and there are enough to level here nonstop.
Arathi Highlands Level 27-28
This grinding location is Alliance only but you’re set for 3-10 per pull and there are enough to level here. Humanoids so you’ll get cloth loot as well.
Dustwallow Marsh Levels 35-36
One of the all time favorite AoE spots from original WoW has been preserved. On Dreadmurk Shore in Dustwallow there are islands of Murlocs. 3-9 per pull, possibly bigger, and they are humanoids for good loot.
Eastern Plaguelands Levels 42-43
One of my favorite spots in Zul’Mashar there are graves on the ground of Infected Mossflayers that will spawn out as you run over them. You can get one big pull of 15, then roam around the edges killing smaller groups until the graves respawn.
Lost Rigger Cove and the pirates are still in play, although not as big and like before, all the pirates are in combat but you can hit them and drop AoEs on them and still get 5-6 at a time.
Swamp of Sorrows 52-53
Misty Reed Strand is full of Murlocs along the coast, you can do 3-5 per pull.
These are some of the best spots available for regular WoW AoE Leveling but there is a complete list of WoW AoE Grinding Spots in my AoE Grinding Guide.
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