You are up against two Marshals. It is relatively easy, all you have to do is buy 15 Assassins and (after selling your stone) spend the rest of the money on Horse Archers. Split the horse archers into two groups, sending one within range of the bottom left Marshal and one within range of the top right Marshal. At the same time, send your assassins to the nearest Marshal at the top left corner of the map. Once the two groups of horse archers reach their opposing castles, patrol them from side to side in front of both castles within range of his keep, and they should fire arrows at the troops there. Careful management of both groups of horse archers is required, quickly move between them with the camera frequently (while waiting for the assassins to arrive) to make sure they are not being attacked by the Marshal's knights. Make sure to briefly move the horse archers away if the Marshal's knights charge, but also be sure to quickly move them back once the threat is over. You could run the knights around with the slighly faster horse archers, firing at them and injuring them. If the knights are heavily injured a group of horse archers may be able to kill them hand to hand. Then once most of the opposing troops are gone, kill the top left Marshal with the assassins (once they eventually get there), before sending them (and the other horse archers) to kill the second Marshal.