Ours was going after a Mystery Wrasse we had just acquired and we thought "Hmm, a $20 fish versus a $170 fish... which one is staying and which one is going?" It was obviously not a hard choice!
We had to dismantle our whole tank to get the slippery jerk. Took all of the other fish and LR out and chased him around till he got tired and slow enough.
He KNEW he was going back to the fish store, I swear he did! He would swim just
outside of the LR and right when we'd go to net him, he'd disappear and then pop his head out of a hole as if to say "Haha!"
I think your best bet might be to start feeding with a net in the water, and over a couple of days the fish will learn that the net holds no threat, and finally, when it seems like you've got a good chance, feed them and then scoop him up.