I'm a rogue and have never had a problem killing things several levels higher if I can get them alone. When I first encountered these shield guards almost every fight ended in me nearly dead until I realized that the red shield hurts you for about 85 dmg everytime you land a hit. Good thing is they stop attacking too. So for rogues, whenever you see the red shield pop up just stop attacking and let your energy regen, once you find this out these guys are easy.