Torch Juggler

Torch Juggler is an achievement related to the seasonal world event called the Midsummer Fire Festival. Completing this achievement is an objective of the meta achievements The Flame Keeper and The Flame Warden.

Materials
are not soulbound, and they are not consumed during juggling; so they can be purchased on the Auction House, or traded from an alt or another player. In addition, you can buy a stack of 5 for from Midsummer Merchants or Midsummer Suppliers, and you also receive 5 from Torch Catching (Horde) / Torch Catching (Alliance).

Strategy
Collect a minimum of 10. Some people have reported doing it with as few as 8, but it is common to have 9 in the air at the same time, so 10 or more is recommended. Note that the tooltip says you must "know how to juggle" to catch them; this means you must have completed Torch Catching.
 * Preparation

Open your Achievements window, find "Torch Juggler" under World Events/Midsummer, and click "Track" to watch your progress.

Make sure you are in Dalaran; you will see no progress if you aren't in Dalaran. Further, you should be somewhere secluded, to decrease lag and minimize interruption by bystanders. Since the torches are a targeted effect, anyone (at random) in the area of effect may catch the torch. Hover over the Game Menu (computer) button on your toolbar or turn on Framerate (FPS) Display (Ctrl-R; or Options, Key Bindings, Miscellaneous Functions, Toggle Framerate Display) and check the FPS in various areas - higher is better. Also note you can usually substantially increase low FPS by standing in a corner, pointing your camera straight down, and zooming all the way in (eliminating required animations).

Drag the torches to an ActionBar key you have mapped to your keyboard (a hotkey).

Hit the hotkey with your non-mouse hand, then click the ground below you. There is almost no cooldown on the torches, so you can toss torches almost as fast as you can click. Using a hotkey means you don't have to move the mouse.
 * Solo Performance

Two or more can complete this achievement together by tossing to each other. This is also more showy, and more subject to lag and interruption by bystanders. More total torches are required, because each player will have torches in the air.
 * Ensemble Performance

Whichever method you choose, you should use the first 15 seconds just to get up to speed. The counter will automatically reset, and you will already have torches in the air; so don't stop after the counter reaches zero - just keep mashing away.
 * Tips & Techniques
 * 1. Rhythm Method: Because of the minimal cooldown, sometimes you will get "Item not ready". Some players have reported success by establishing a rhythm to minimize or eliminate those errors, theoretically optimizing the delivery speed.  However, not each torch is thrown to the same height, resulting in different hang times, which means they do not return to your bags at regular intervals.  In addition, there also appears to be some inconsistency in the cooldown: sometimes there appears to be no cooldown at all; other times there appears to be a fraction-of-a-second cooldown.
 * 2. Speed Freak Method: The most common method, just alternate between the hotkey and mouse as fast as you can. This results in a lot of "Item not ready" errors, but minimizes the effect of the cooldown inconsistency.
 * 3. Spaz Method: Try not to alternate between pressing the hotkey and clicking the mouse button, rather do both at the same time. Some players believe this results in higher speeds, but some may find it difficult to press both keys together.

Try them and see which works best for you; but whichever does, persistence and high FPS are the key.

Isolated locations
Some sample isolated locations
 * Archmage Vargoth's Retreat
 * The Beer Garden behind A Hero's Welcome
 * Dalaran Floating Rocks, accompanying the city and considered part of Dalaran.