If there is a shed or other enclosure nearby then get a group of 3+ people (the more the better) and have everyone SLOWLY herd the duck inside.
SLOWLy needs a lot of emphasis. The slower you go, the faster you'll catch the duck. The moment he realizes he's being chased he will go everywhere but where you're herding him.
If there isn't an enclosure then hopefully there is a stout tree nearby. Get some sort of large cheap net (like a volleyball net) and some thin, dull colored rope. Cut the rope and tie a section to each corner and in the middle edges, then toss all the separate ropes over a tree limb and test it out to make sure that all the edges raise at the same time when you pull on the rope (bonus: toss something that weighs 5-10lbs in the middle to see how it bends).
Once you have that figured out set up your trap and get out the duck treats. Set up the net and drop some treats around it, then leave it alone for an hour or two for the duck to become familiar with. After that all you have to do is have someone dropping treats in the middle of the net with someone else holding the lines. When the duck is in the middle bring it up as fast as you can.
It's the easiest way to catch a crazy duck or goose out in the open. We've also played around with varients by having the net underwater, tugged at from different ways, and braided or looped through a metal eye that keeps everything raising together.