What the achievement asks
Free 200 souls in The Maw. Ten points, account-wide, both factions.
Straightforward, once you know the one rule that is written nowhere in the game.
The rule nobody tells you
Souls only count while the weekly quest is in your log. Free five without it and you get nothing: no progress, no message, no hint that anything went wrong.
The quest is Return Lost Souls, taken weekly from your Covenant Sanctum. It only asks for five, but leave it in your log and keep going: the achievement keeps counting well past what the quest wants.
If your counter has been stuck at zero for a while, this is why. Go and pick up the weekly first.
What a soul looks like
They are called Imprisoned, Aimless or Wounded Souls, and they are scattered across the whole zone. Each shows on the minimap as a small ghost icon.
Some float free and only need a click. Others are sealed inside stone prisons: break the prison, then right-click the soul inside.
The fast way
Zovaal's Cauldron has a cluster of stone prisons packed close together, and it is where people go to do this in one sitting.
Two things make it quick. Join a raid group that is sitting in the zone waiting on something else, so the prisons keep being broken around you. And split the work: someone smashes the prisons while you follow behind clicking the souls. Two hundred goes in about an hour that way.
The Maw is watching
Freeing souls raises the Eye of the Jailer. It climbs slowly here compared to fighting, but it climbs, and at the higher tiers it will start killing you outright while you are still clicking.
If you are already deep into the tiers before you start, you will die repeatedly, and the answer is more hands rather than more caution: with enough people breaking prisons you finish before the pressure matters.
Or get it without trying
If you are working through Torghast anyway, you will pick up souls all the way. People finishing Tower Ranger report arriving at roughly two hundred without ever making a trip for them.