For this mission, I believe the rangers, archers and halberdiers refer entirely to our dragoon-trained retinue of those types. There's no place for infantry in an enterprise such as this. Hopefully we have spare mounts, as well, since we'll be doing hard riding.
First thing, let's get out of the agreed area before being spotted. The agreed area needs to be off the radar and our attacks will help deflect attention elsewhere as the duke gets into position.
Circle the horde to the west, find a good ambush spot for the archers and rangers, supported by most halberdiers; with the small unthreatening remainder, we clomp leisurely up to the enemy like bumbling fools to draw off whatever light cavalry screen into the prepared ambush, then ride hard when we get some attention.
We can rinse and repeat a little, then begin scouting for forage parties. If there's no appreciable cavalry force, then simply proceed to ranging for small groups returning with food. They'll probably take carts and go along roads, so sit on those roads in a good concealed position. An army of 15 thousands needs 22.5 tons of food each day, and that food is distributed across the countryside in need of gathering, but eventually all village paths lead down into the main roads, so that's the kind of traffic you'll see on those roads. Fire and sword, then shift position, and then fire and sword.