Kill 50 rookery whelps within 15 seconds.
Title Reward: <Name> Jenkins
This achievement will become a Legacy achievement in Warlords Of Draenor, it will get unobtainable, so get it while you can!
The title will though still be obtainable through the new achievement: Leeeeeeeeeeeeeroy...?
The minimum level requirement for characters to obtain this achievement is level 48, since that is the minimum level restriction for entering the UBRS instance. The best and simplest method - and really the most practical at this point in the life of the game - is using a level 90 to carry you to victory. Level 81+ is good enough, but this late in the expansion you're more likely to find a bored level 90 than a bored level 81+.
The whelpling eggs are located in The Rookery, which is the room just past the first boss of UBRS, Pyroguard Emberseer. He must be defeated before the door to the next room in the dungeon will unlock. Be very careful to not touch any eggs just yet. The range is very deceiving. Kill the two Rage Talon Dragonspawn just inside the rookery and the two Rage Talon Flamescale. To spawn the whelplings, simply touch the eggs as you run around the room. The eggs hatch after a few moments, so do not immediately use damage AoE spells. Spawn 50 whelplings (just run around and spawn all of the eggs) then give them time to catch up to you. Proceed to AoE them to death in under 15 seconds. The classes with the easiest AoE abilities are mages, warlocks, and monks.
Note 1: At level 90, it is highly likely that you'll have to run back to pick up groups because the whelplings take a second to after hatching to aggro onto the nearest player - or the lowbie character, if you're carrying one. If you are indeed carrying someone for this achievement (or just for the dungeon), have them stand in the back of the hallway leading into the rookery; and pause in your running every time you hatch a group.
Note 2: Be sure to do this achievement while on a computer system that can handle displaying that many mobs and abilities in an area. If you are unsure, just set your video settings (Game Menu -> System -> Graphics) to the Low setting and don't have your camera zoomed out too far.
It is not necessary to acquire this achievement for each character since titles are now Battle.net account-wide with Mists of Pandaria; the title may be used by even level 1 characters. However, many players do enjoy collecting achievements anyway, especially ones that will no longer be possible to obtain.