|Equal Opportunity Eviscerator
Kill a Pirate and a Knight with a single axe swing.
How to: The axe has a decent enough range to make hitting two people a reasonable task with a horizontal swing, but killing two people? Let alone a Pirate and a Knight? The most likely scenario in which this will occur is you stumbling upon an inept Skirmisher flailing against an incompetent Archer, and they both more or less ignore you as you take their heads off.
Another potential scenario is when they're fighting over an objective, such as when a Kamikaze Kaptain has hobbled into the Knights' throne room in Island, and a Knight that you were fighting just happens to get too close to him as he tries to pick up a chest.
This achievement just can't be engineered short of deliberately farming it with a couple of friends (or friendly strangers). The best strategy is to remember that it exists, so you never miss an opportunity to try and get it.