Although the above solutions are good, they predominantly focus on getting it in a new or first run. Now with New Game+ and Scene Selection, this just overcomplicates things. I got this on a New Game+.
1) Equip the Skills Firearm Lv. (X) and Quick Reload
2) Either get to the point in Leon's campaign where the alter in the cathedral opens from a New Game+, OR go to Chapter Select and select 2-3 Cathedral and get to this point.
3) Unload your Lightning Hawk onto the beastie.
It really was as simple as that, it went down in a matter of seconds, with the achievement popping after its death animation.
Comments, questions etc. welcome as always
-PP70