Just in case you are unaware, the majority of mobs in an instance do not de-aggro or "leash". So you can round up just about the entire instance and AoE them down. No problem.
But that's not the case out in the open world of Azeroth. There are some good spots where mobs are grouped and whatnot, but most of the greens they drop just aren't that great. The mobs that drop the best stuff - like Jade plate - aren't that well grouped. Can't round them up too well with damaging abilities either at max level because you one-shot them.
Here is your solution:
Druids, stay out of form and do the same with Faerie Fire. Tooltip has changed since the recent patch, but the small amount of damage it does is utterly insignificant and will not one shot mobs.
Not any one of these classes/don't have one/don't want to farm with it... hope isn't lost. Warlocks, use a dot spell that does not do initial damage - quickly tab between mobs and apply this dot before it ticks damage. (I am assuming the dot can go only on one target at a time, thus gaining aggro while dealing no damage.) Warriors, Paladins, and DK's, use your taunt. Paladins and DKs can even use consectrate/death and decay. Hunters, use distracting shot and slowing traps. Mages, I do not have one of you and thus do not have a picture, but you can use Spellsteal in the same manner as Purge, Faerie Fire and Dispel Magic. Rogues... Nobody loves you. Please help yourself to a chocolate chip cookie.
Now that you know how, where do you go? Un'Goro. Silithus. Winterspring. Best bet is Un'Goro and the Stegadons, as elite mobs have a higher chance of dropping greens. It'll still be long, tired, and somewhat unrewarding. Farming is not for everyone, but it gets you some gems if you persist.