Three musketeers flanking achievement - Civ5

Playing as Napoleon/French in Civ5, I am struggling to get the "Three Musketeers" achievement. I suspect the answer is somewhere in what is considered to be "flanking" (How do you flank an enemy?), but not quite sure. Has anyone achieved this, and know how to get it?

A flanking bonus in Civ V is given when you attack an enemy unit while one of your other units is adjacent to it. This represents the tactical value of surrounding the enemy.

To do this achievement, you need to be playing as France and to have three musketeer units. Naming them is optional but for this example we'll call them Porthos, Athos and Aramis. You'll also need an enemy unit- city states tend to have some around. Using a city state for this also means you don't need to declare war until you're in position.

Your victim needs to be a military unit and should be weak enough that your musketeers can kill it. Most pre-gunpowder units will do. Better still, bring ranged units along to weaken the enemy.

Use the ranged units to weaken the enemy, then move Athos and Aramis next to it. Finally, have Porthos attack and kill it. He should get a bonus from the musketeers and you should get the achievement.