The Longbow are fine. They are not too hard, they just present a different challenge than other mobs. Why is it that I play a necro MM (whose pets are very weak to fire) and I have never had a problem with Longbow Flamethrowers, even in multiples? Just plan your attacks, think about what you're doing and don't try to take on more than you can handle.
BTW, don't forget to play dirty, we're villains for cryin out loud.