Bounce a ball with a hatchling 25 times in a row without dropping it.
You need to be affiliated with the Order of the Cloud Serpents (located in The Arboretum, The Jade Forest) in order to own a hatchling. To do this, your character must have reached level 90 to unlock the starting quest, which requires you to defend the wild cloud serpents at Windward Isle from the Slitherscale reptiles. Soon, you will get a quest that lets you choose one of three eggs. Each has a different color (blue, green, or yellow) that corresponds to what color your baby serpent will be.
Once the critter comes out of the egg, the daily quests are unlocked. One of them is Catch!, which lends you a Bouncy Ball that you can use to play with your pet.
As you and your hatchling throws the ball in the air, a big and dark circle of shadow – more distinct than the smaller and lighter shadow casted by the ball – appears on the ground to indicate where the bouncy ball will land. These spots are where you and your hatchling are going to hop onto one after the other; hence the achievement title Shadow Hopper.
How to complete Shadow Hopper
Whereas the Catch! daily quest counts every successful catch of the ball, you can only complete the achievement if you repeat the process 25 times without dropping the ball.
The most challenging part is finding a sweet spot where you and your pet can play without interruption. Distancing yourself from other players who are also playing catch is best, as you might be confusing each other with so many balls thrown in the air.
Another problem is the occasional bug with the geometry of the terrain; sometimes the ball lands in a location that is impossible to go to. Also, avoid any water.
The Juggler buff appears right after your first successful catch. It will indicate how many more times you need to throw and catch the ball to complete the feat. If you miss a catch, you need to start over by removing the buff and throwing the Bouncy Ball once again.
Once your reputation with the Order rises to Revered, your hatchling will grow and no longer be able to participate in Catch! If you still have not accomplished the achievement by then, you can simply ask the Serpent Master, Elder Anli, to borrow one of the other hatchlings and play with that critter instead.
As sometimes the ball is thrown quite far from you, you will need to use some speed increase skills or spells to help you. What I personally did was to use my flying mount. But to do this all the way to 25 catches, you must only mount RIGHT AFTER your pet has thrown the ball and it’s your turn to catch (note: you get dismounted every time you throw the ball). Mounting beforehand has a likely chance to distract your pet; he will return right by your side instead of throwing the ball.
If you’re lucky, the count gets bugged and will still stack towards 25 despite any misses you made during the game.