Actually I found that if I get the crossbow to max range, and I wait to start spamming until the first orc hits my arrows, it works out great. Just kill all the orcs before they make it to your hobbits, so your hobbits don't slow down
Demonjin is absolutely right. Mission 6 is a very quick 6k+ xp. Just start pumping hobbits right from the start, don't even bother using your crossbow, and it's an easy, quick win.