Ottoman Mastery By Wing and Hoof

I am stuck. It calls for defeating 15 ranged enemies.
I have played it several times, Arrange a lot of Sipahis. Attack using Fortitude. Kill a bunch of archers (English) and yet cannot get past this. What Am I missing?

Have done the same againts the Mongols killing the mounted archers.

My only guess on what you are missing is that you need to kill 15 during a single game. I think the best way to do so is start a skirmish against hardest English ai and not finish the game too soon. Let it rebuild an army several times just to make sure.

Oh and one more thing. If I recall correctly Masteries don’t work if you save-load the game. So make it at one go.