Bring a distance weapon of some kind to the last level. The mission assumes you have one equipped. If you do not bring one then prepare to die a couple of times to complete the mission.
A really powerful sword may work. Still, pack a distance weapon to save yourself a lot of headache.
How long of a distance? Probably not necessary, but if there's guns that aren't far enough, we should know.
Powerful as in, damage? Or powerful like da ranged charge?
Or am I missing a point and you are refering to ranged kiting of mobs?