This requires you to get an 'Incoming!' skill kill on the Coastline map. In the middle of the map you will see a large rock. I guess you could say, compared to the rest of the landscape it's a mountain. Anyway, the point is it's hard to miss which rock you require. An 'Incoming!' skill kill is where you kill someone with an air stomp. Passing through this rock is a tunnel, so you want to watch for enemies entering and exiting underneath you. When someone is below you, press to jump and at the height of your jump hold to initiate the air stomp. If you've timed it correctly, the air stomp will land on top of an enemy killing him for your achievement. This can be obtained in any game mode.