Agreed with the other posters. Cold is sometimes acceptable like CO2, again, *if* the animal is small enough. This is especially common in lizards. It's not appropriate for a full sized duck. CD/Broomsticking is by far the easiest.
CONTENT WARNING, broomsticking of a chicken, chicken death
This video is a great example. You can see how PHYSICALLY anyone who can stand and touch their toes can do this. If you do it too aggressively the head can come off - that can be visually distressing but has no impact on the quality of the euthanasia and it's fine. I would err on the side of being too aggressive than not aggressive enough. PHYSICALLY you can do this on your own. It really doesn't take much muscle. If you're on your own then mentally it's probably the best option too, but again, don't stop half way. Once you step on the broomstick you MUST follow through.
But as you can see, from alive and comfortable at 1:31 to the spice being completely dislocated from the skull at 1:33 it's a VERY fast process.