Online: roadkill an enemy with any helicopter
An easy way to get this achievement is by using a UAV station while playing online. Fly up high until you find yourself some lonely group of troops or a lone sniper. Once you find yourself some enemies, fly down low and hit your enemy with some speed. Should be an instant kill. Most enemies don't expect a helicopter inside of a building either.
Alternatively, a slightly harder way is to take control of a full sized helicopter and just crash/fly low into an enemy.