The biggest advantage of the xbow is obviously the ability to hold your shot without losing accuracy, so play to that strength.
If someone without a shield is zig-zagging towards you, don't move your reticule just wait till they run into it.
You should prioritise cav since you're the best equipped to take them down; do this by waiting till they're running in a straight-ish line perpendicular to you, lead them by a good amount and fire. For slow moving/nearby/shieldless cav, aim at the rider instead.
On flat ground, go for head shots, especially when shooting into a large mob. On maps where you know where everyone runs to at the start of the round, try to be in a position to shoot into them as they get there, because there's almost no chance they can all get out of the way.
Going 1v1 with an archer on open ground or at close to medium range probably won't go your way. Wait till they're distracted and shooting at someone else. 100% of archers stop to shoot, so wait until they do so and then head shot them. You'll only get one shot before they realise you're there, and hitting an archer who is actively trying to dodge you is not worth the effort.