The Light Crossbow
It should be noted that killing a full-health enemy in one shot is a rarity with the Light Crossbow. Knights in particular will take multiple shots to take down. Also note that the Light Crossbow has the same damage input as the Warbow, meaning that it would take longer to take another shot from the Light Crossbow since you have to reload.
Although the reload is shorter, the reduced damage makes the Light Crossbow a really hard weapon to use effectively. Generally speaking, the light crossbow is for picking off already weakened targets (which some may deem as killstealing) or for a fairly dependable hit on a target prior to melee combat (though be prepared for their arrival).
The defining feature of the crossbow is it's lengthy reload. As you press R to begin the reload, your camera angle pans straight to the ground - Your character must anchor the crossbow with his foot, pull back the bow, and insert the bolt.
DO NOT switch weapons untill the bolt is firmly inserted and the crossbow is raised. If you switch prematurely, you will be left with an empty crossbow.
As a crossbowman you're automatically equipped with a Pavise Shield on your back; which can be used as cover while reloading. If you need to reload while in combat with an enemy archer, put your back to the enemy and crouch. The Pavise will cover your character while you finish the crossbow's lengthy reload.
You may also press 3 to place the Pavise Shield on the ground, making useful cover.
You may not use the Pavise like a conventional shield.