use splitting arrows or magic blast;) against them and rush attack to kill the last one
The problem with archers is they tend to come in groups and can stun you for 5 seconds or so at a time, and will often do so back-to-back, leaving you stunned for 5-20 seconds. Rushing into a group of archers on Nightmare will often mean certain death. The best strategy I've found for dealing with them is to get their attention, then run behind a wall or post or something. They'll be forced to run around into close range where you can usually finish them one at a time. Summoning your creature might also help.
