Adding on to my last post, leveling archery works best when you are on horseback. Even using a hunting bow and normal arrows it is way easier to get kills because of the mobility a horse affords.
I start off every new game by getting a low level bow and hiring a Vlandian squire or two. These guys kind of suck, so while they are faffing about with the looters, I am lurking nearby with my trusty bow, killing the looters that the squires are unable to hit. I usually get most of the kills and my bow skill rises very rapidly. Eventually the squires get a few kills and level up, but by that time I have 50 archery or so.
Stay mounted if you are trying to grind up some archery skill. There will be plenty of time to run around like peasant after you have mastered the bow. I hope this will clarify my earlier comments.
This is good advice.
Mounted archery is the optimum way to level both bow & riding, I usually start most playthroughs grinding looters on horseback. Headshots while riding at max speed, while tricky, work best.