Connecting with the head of your target through good aiming is critical to the solo PvE archer. However, it’s quite a difficult thing to master! Practice, practice, practice. Here are my tips:
Method #1 is to shoot directly at non-moving targets, like those standing, sleeping or slow-moving mobs. This is the easiest one! You don’t have to account for time to target, only arrow drop. When starting out focus on center-mass chest hits. Then, once you get the hang of arrow drop you can raise it up and start focusing on the head. This can be used against moving targets when the mob turns around – they stay in place for a quick second during the turn (like the pirates that just wander back and forth). Sometimes you are close enough to a moving target to just shoot it directly.
Method #2 is for moving targets, keep your reticle fixed near the target and release when the target moves into your reticle. This works best against mobs that follow a predictable pattern like the wandering pirates or such. It’s not great for animals as they vary their movement a lot. You do have to account for time to target and arrow drop. I use this one a lot if the mob is running directly toward me, usually on my second shot.
Method #3 is to track your reticle with the movement of the target and release when you are confident you have a pattern match. This takes good computer mouse control. One has to “lean forward” a bit with the aiming if the mouse is slow. If you get really good at this technique, you can try hitting 3 different pirates lined up sideways with rapid shot. Very small movements with the mouse can make big changes in the arrow trajectory.
I often find I am using a combination of methods. If the target is moving directly toward me or away from me, I use method #2. If it’s moving sideways or slantways, I use #3. Time to target delays and arrow drop are something you learn by being observant. I’ve burned though a lot of arrows trying to make ultra-long-distance shots, which helped me learn.
Watch out for engaging targets as they move near trees and fences, seems like my arrow always hits the tree. I’ve learned to avoid those shots.
Good luck aiming for those head shots! Concussion provides a 20% damage boost and Hawkeye provides a 10% heal with each head shot. At 200 dexterity points there is a 10% headshot damage boost. Why hit anywhere else 
What are your tips?