Merlin drew his bow back, aiming it at one of the horses in the front carriages.
His first goal would be to prevent the carriages from running away. If he started picking off the guards, it was likely that the carriages would continue to their destination while leaving the guards as a distraction.
For this task, he only had to kill one of the two horses each, as the dead horse's corpse would serve as a severe inconvenience that would hopefully prevent the other horse from moving the wagon much, if at all.
As the horses rode closer, Merlin's eyes locked onto the clover insignia on the covered wagons, which was all he needed as confirmation to let the first arrow fly. The arrow flew out of the treeline, embedding itself in the head of one of the horses tugging along the leading wagon.