When you start at the begining of Explore Aslan's How, there are three passageways, you have to do the one straight ahead of you first to unlock the second one which will be to your right as soon as it teleports you back. Lower the bridge and cross, follow commands till you come to a stone circle, use the circle thing to move it around ONCE, then use the lever to change the circle, than move it around with the circle thing again and keep doing that till a cutscene appears, go down the stairs.
Now you should be infront of a pond, walk into it and grab the lever needed, and go up top and place it and active the lever will which open up the door. Lucy only has the horse so when you exit get as close as you can to the two men on horses (barricade in the way) and fire an arrow at them which will make them come, steal the horses and keep going over the barricades (have to push a button to go over them).
After every jump you will encounter a couple men, just shoot them, the best way imo is RB A which will fire 3 arrows at them. After that you will get to a bridge, cross and you will come upon a group of men and a big guy, the men are easy just fire or use your sword, for the big guy you have to keep hitting him with your sword which will take probably 15 hits.
Now grab the lever on the ground infront of the gate and place it in the holder and operate it, which will open the door. Go in and your encounter MORE soliders, kill them (suggest you hop back on horse first so you get closer to Kill 20 Soliders While On Horseback) after their all dead go to the turning stile while on your horse and push it which after awhile will open another gate, just gallop/walk down that small passageway and you will get the achievement.
Story related.