What the achievement asks
Summon 25 Massive Lunkers at a Large Fish Sighting. Ten points.
The criterion reads Use Ominous Conchs at a Large Fish Sighting, which is the same thing said backwards.
Only your own summons count
A Massive Lunker is summoned with 5 Ominous Conch at a Large Fish Sighting, the spot inside a fishing hole marked with a fish icon on your map. Twenty-five of them is 125 conch, and there is no way round the number.
What does not count: lunkers summoned by other players, lunkers summoned by anyone in your party, and lunkers summoned by the Looking for Lunkers buff you get from looting a conch, even when the buff was yours. Only the ones you spend five conch on yourself.
That is what separates this from the earlier lunker achievements, which count everything, and it is why people arrive here thinking they are nearly done.
This achievement only appears once the earlier Massive Lunker achievements are behind you. If it is not showing in your list, those are what to do first. Ominous Conch are also soulbound, so nobody can hand you a stack, and it is worth doing all twenty-five on the same character.
Where the 125 conch come from
Tuskarr Tackleboxes are the fast route. Thirty-five conch in half an hour has been reported against roughly one conch every ninety seconds from fishing. The boxes are scattered across the zones and lie thick in the Restless Wetlands in The Waking Shores, and a quiet zone or a war mode toggle keeps them from being picked clean ahead of you.
You must have the Iskaaran Harpoon in your bags for conch to appear in the tackleboxes at all. Without it you can open a hundred and find none.
Fishing works too. Open water inside the marked fishing hole areas has a far better conch rate than fishing anywhere else, at roughly one per one to three minutes. The pond in Loamm, in Zaralek Cavern, is the spot people name for sitting still and grinding.
Spend them in a group
Look in the group finder for a group with Lunker in the name before you go. Everyone stacks at one sighting and throws conch, and although only your own summons count towards this achievement, lunkers summoned by other people still drop conch. A busy sighting therefore refills your supply while you are spending it, which is why players who did it in a group report about three hours from start to finish.
Sightings are marked on the map with a fish icon inside an active fishing hole. Since only two holes run in a given week, this shares its calendar with Taking From Nature: bring conch every time you visit a hole for that achievement rather than making separate trips.
Where it fits
One of eight under Army of the Fed, a piece of A World Awoken.