This one will easily be the hardest of the four to level up to level 10. It is best if you tackle it in the French campaign as there is a little more leeway for infantry units to dominate, where as in the English campaign, most of the missions rely on the longbows of the archers to keep the French at bay.
It may even be best to simply wait until you have finished both campaigns, load up a new French campaign on easy, and let the infantry be the focus.
Units level based on the amount of kills they get in any given mission without fail. Capturing or doing objectives may contribute to their possible exp boost, but first and foremost is the number of kills they get. You will get this achievement when any infantry unit reaches level 10.