1. Find a place where there's a flying/non-attached platform, I chose Icecrown, about the middle, where the undead are.
2. Shoot a mob, make sure he can't get to you.
3. Setup an auto-clicker to click the shoot button once every 30s.
4. ???
5. Profit
It's 30s because you don't get weapon skills if the mob is pulled/can't attack/bugged. But you can get the initial shot skill up.
OH, and at first, the mobs will leave combat after about 12s, but after then 3rd or 4th shot, it changes to about 25s, haven't timed it exactly.
I did this overnight/during the day to level up my low level crossbow/gun skills. After you get to 390 though, it would probably be safer/more beneficial to just level 390-400 the real way, but if you wanted, you could still do it that way.
Auto-clickers are all over the forums, if needed, I can post one though.