Juggle 40 torches in 15 seconds in Dalaran.
It is possible to earn this achievement outside of the Midsummer Festival holiday event because Juggling Torches do not have a duration. However, your character must have completed "Torch Tossing" (A) / (H) and "Torch Catching."
1] Acquire ten Juggling Torch . (I only needed six but you may need more if you have higher latency. Better to be on the safe side.) They can be purchased from the holiday vendors during the holiday event, from completing the holiday quest "Torch Catching" (A) / (H) and the follow up daily quest "More Torch Catching" (A) / (H), or bought from the Auction House and fellow players.
2] Get to Dalaran. There is a portal to Dalaran in both Pandaren shrines, or you can ask a mage. (Um, a word of caution: watch the mage's cast bar to make certain they are casting Portal: Dalaran - Northrend and not Ancient Portal: Dalaran . The latter will teleport you to Dalaran Crater in Hillsbrad Foothills - about a hundred yards above the ground.)
3] Find a place in Dalaran where there aren't many players. If you can fly, then this becomes easier. I picked an empty rooftop on my rogue.
4] Put Juggling Torch on your action bar. I chose to bind it to '3', but you can place it wherever is best for you.
5] Tilt your camera until you are looking straight down on your character.
6] Zoom in your camera view. How far you may want to zoom in is dependent on your latency and GPU (video card). I zoomed my camera in until my character was just shy of becoming translucent.
6a] If you feel you need to, turn down your graphics settings. (Game Menu -> System -> Graphics. If you aren't sure about what setting does what, just use the Graphics slider to lower your settings. Make certain to click the "Okay" button when you are finished.)
7] Press one time whatever key/button is bound to Juggling Torch. Position the targeting circle over your character.
8] I hope your mouse is up to the challenge because now you are going to press that action button and click your left-mouse button for all you are worth. Keep going until you see the achievement flash at the bottom of your screen.*
* Patch 5.3 inadvertently broke achievements that depend upon the game using a timer to track criteria. If you fail the achievement, the timer will not reset; you will have to wait until Blizzard fixes the problem before any additional attempts may be made.