Like all the other solutions outthere this one also is boring almost annoying.
The easiest way is to play the Arena till wave 30,
there will be a zombie carrying a ex. weapon all the time.
If you have trouble getting to wave 30, just hop in a few lobbies and there will be someone who is kind enough to dupe a "One Hit Kill Weapon" and you're good to go.
And, if you need 5 weapons it's 5 times wave 30, which will result in a heavy amount of Exp. and $$$ hwat makes level50/60 way easier.