You can't do anything other than hit more and do more damage. The reason xp is lower on arty is because you split it with the spotter. Price paid for sitting at the back in safety. If you ever direct shoot someone that you spot yourself you get more xp. It is not advisable to do this except maybe on city maps and go TD mode in a fast arty like the Hummel can somtimes work on those maps. As for best place to shoot/aim with arty. It is just behind the target. Not on target. Due to firing arcs on most arty the shell will hit them more often doing this and has more chance of hitting them on top. Less of an issue with high arc loer tier arty, but high tier you should aim just a small amount behind the target.