One down, on the Ground!

Airborn coffins are an affront to gravity. Use your equipment to avenge Mother Nature! About halfway through the cemetary, you'll come upon a large circular courtyard with a spooky tree in the center. When you do, immediately look up and shoot down a flying coffin. If you miss, you can try again when the other two Black Slime Monsters pop up, or by replaying the mission later. This portion of the level is right at the beginning of the checkpoint, so reloading to try it multiple times to get this achievement won't take a long time.