I know that you specifically asked about crossbows, and much of the above information is very useful. However, combine a good runearm with that a nice repeater for even better damage, and remember that many runearms are bound to account, so you can farm them with another toon and have them ready as you level up! Also, if you have enough crafting and House Cannith favor, you can make the Shard of Masterful Craftsmanship to lower the level by 2, which works on repeaters and runearms!