Leeeeeeeeeeeeeroy!

Leeeeeeeeeeeeeroy! is a Legacy achievement earned for killing 50 in the Upper Blackrock Spire instance in 15 seconds.

Location
This achievement can only be completed in Upper Blackrock Spire. It doesn't have to be the rookery, it can be any location with eggs. The easiest spot is the room you fight Rend Blackhand in. He stands on a ledge and calls for waves of enemies to attack the individual in the large room. If you notice the small room these waves appear is full of eggs all of which respawn. Simply run in there and get the achievement easily.

At level 80, it is possible to run over an egg and run out of the reduced aggro radius of the drake before it hatches.

It could be important to note that, so long as you participate in the fight, if one member of the group gets the achievement the rest will too.

Trivia
As of June 2012, the actual Leeroy Jenkins character does not have this achievement, despite being level 85 and having defeated Deathwing.