Far and away, the best method is in horde. The opponent just stands there, and when their health is less than (around) 250, fire it.
The harder part is getting the Hammer. It's an Epic Arena 4 drop, so you'll have to wait until you get 900+ cogs before you start seeing them in crates (although you might get lucky sooner).
The easiest method of all is to play horde with someone you know who has the Hammer. If they use it and win on the final shot, you get the achievement as well!