I'd like to use my darts on nex but have not even been in the GWD in years upon years. What monsters and setup would get me the 4 key pieces easily, and where are they?

Zammy/Sara stuff near the entrance of GWD, Aviansies for Arma and Goblins/Hobgoblins/Ogres (I think?) for Bandos. Normal melee gear for Zammy/Sara/Bandos and range for Arma. Make sure to be potted and using a range boosting pray in order to beat the bots (Should obviously be potted/praying for the other stuff too, but for Arma it's much more important).

 

EDIT: Or do you need to be inside the lairs to get the key? I honestly forgot. If so, the same still applies-ish.

Yes, only monsters in the lairs drop key pieces.

 

Good luck!

The easiest way to get them is to enter each 'stronghold' as thats only place keys drop and find a cluster of the weakest monster to stomp on.

 

So like imps for Zammy

Hobgoblins(?) for Bandos

etc.

Can you only get key parts from monsters within their encampments (for example, sara mages in sara encampment will drop them, but sara mages in the armadyl camp won't?

Can you only get key parts from monsters within their encampments (for example, sara mages in sara encampment will drop them, but sara mages in the armadyl camp won't?

Yes.

Each god's piece of the key only drops from their followers and only within their 'stronghold' area.

And you can only get one fragment at a time, don't waste time like I did looking for spares.