What the achievement asks
Defeat Captain Reykal, the Sundered Flame leader, during a Researchers Under Fire event in Zaralek Cavern. Five points.
This one is not account-wide, whatever the branch above it suggests. It has to be earned by the character that does it, so if you are spreading the meta across alts, do it on the one that will finish.
When the event runs
Researchers Under Fire takes place in the middle of Zaralek Cavern every hour, at the half hour, and lasts about ten minutes. It is marked on the zone map while it is running. Reykal is the finale, and credit lands when he dies.
The reason this is not the ten-minute job it used to be
While the patch was current, thirty people turned up and Reykal fell over. Today the event is often near-empty, and what is left is a leader flanked by five or six elites with many times your health, in an event Blizzard designed for a crowd.
Do not assume you can solo it. If nobody else is around, use the group finder: people list custom groups for this event and for the rest of A World Awoken every reset. A handful of players is enough, and the event is short enough that organising one is barely an interruption.
Arriving a few minutes before the half hour matters more here than usual, because the ten minutes are tight when the group is thin and a slow start can mean Reykal never spawns.
Go on reset day
The same event is the best source of Unearthed Fragrant Coins, which is all The Smell of Money is made of, and your first completion each week pays a far bigger pouch than the rest. Doing this achievement on reset day therefore hands you a large chunk of a second one for free.
Where it fits
One of the pieces of Across the Isles, which rewards the Stormtouched Bruffalon and is itself a piece of A World Awoken.