What the achievement asks
Become the Hand of Fate and finish the Nazmir world quest Getting Out of Hand in twenty seconds, without being in a group. Forty kills, twenty seconds.
You pick up a Scroll of Fate's Hand, which puts an extra action button on your bar. Using it turns you into a giant hand with three abilities, and the only one that matters is the second, a slam that flattens everything around you. One slam on a big enough pile is the whole achievement.
Eighteen percent of players have this one. It is the hardest of the Nazmir set, and almost none of that difficulty is in the fight.
Do not wait for the world quest
The world quest is not the only way in. The Horde Nazmir campaign quest Forcing Fate's Hand puts you in exactly the same encounter, and you can abandon and retake it as many times as you like.
This is the single most useful thing to know about this achievement, and it changes it from a once-a-month lottery into an afternoon's work.
The campaign quest asks for fifty kills instead of forty, but the achievement fires at forty, so the extra ten cost you nothing. If you fail, drop the quest, pick it back up, and try again. No waiting, no alts burned, no crowd.
Getting there on a fresh Horde character takes twenty to thirty minutes: start the Battle for Azeroth introduction in Orgrimmar, take the Zandalar chain to Princess Talanji, choose Nazmir on her map table, and quest until Forcing Fate's Hand appears. It works on a trial character too.
The world quest itself surfaces rarely and stays about a day, so if it happens to be up, take it. But it is no longer worth waiting weeks for.
Where it happens
North-east Nazmir, at Torga's Rest, behind the enormous dead turtle.
The area is a scatter of raised platforms with skeleton packs on them and looser packs on the ground between. A pack is roughly ten to twelve skeletons, so you need three or four packs stacked together to clear forty in one slam.
The real problem: the packs will not follow you
Everything that makes this achievement hard comes down to one thing. The skeletons leash very quickly. Walk more than a short distance from where you pulled them and they turn round and go home, so the old approach of running a loop and collecting four packs behind you no longer works as it once did.
Worse, the scroll has a cast time, and any hit interrupts it. So you cannot simply gather a mob of angry skeletons and transform in the middle of them.
Four ways round it, in order of how reliably they work:
A Brewmaster monk and the Black Ox Statue
The most dependable answer by a wide margin. Take the Summon Black Ox Statue talent, drop the statue in the middle of the packs, and pull each pack onto it. The statue holds their attention for fifteen minutes, so nothing chases you, nothing interrupts your cast, and you can take as long as you like to build the pile.
Two things to get right: it is the Ox Statue, not the Ox Effigy, and it sits in the left side of the Brewmaster tree. And take off any trinket or gear that deals damage on its own, or you will start killing the pile before you are ready.
Any immunity
The cast only needs a few seconds of peace. Pull your packs, pop an immunity, cast the scroll while nothing can touch you, then slam. Divine Shield, Ice Block, Netherwalk, Turtle: they all do the job.
A friend who is not in your group
The achievement forbids a group, not a helper. Someone else can gather and hold the packs for you while you stand clear, transform in peace, and slam. Make sure you are on the same shard and that you leave the group before starting.
Walk in inches
If you have none of the above: mobs keep chasing as long as they are actually hitting you. Move a step at a time, let them stay in contact, and you can drag a surprising number across the area. It is slow and it works.
The slam, and the trick after it
Transform, then hit the second ability. On a full pile that is often thirty-five kills in one press, and a second pack finishes the job.
There is also a shortcut worth knowing: after transforming you can leave the vehicle and go back to being yourself, then kill the rest with your own abilities. At current levels a max-level character clears the remaining skeletons faster than the hand does, and it sidesteps the clumsy hand controls entirely.
Track the achievement
Track it before you start and you get a twenty second countdown on screen. Without it you are guessing, and people routinely fail by two seconds while convinced they were well inside.
There is a long-running argument about when the clock actually starts: on the transform, or on the first mob you aggro. Reports go both ways and neither side has ever settled it. Assume the worst, which costs you nothing: once you touch the first pack, do not dawdle. Gather, transform, slam.
The other players
When the world quest is up, the area fills with people doing it normally, and they will kill the pile you spent two minutes building. There is no way to reserve mobs and no way to reset your attempt.
Two answers. Turn on War Mode, which usually empties the area completely. Or go at an off hour. Or, best of all, use the campaign quest instead and skip the crowd entirely.
One more thing worth knowing about the area: the packs do not all respawn at once. After a wipe or a failed attempt, give it around five minutes before you count on four full packs being there.