On Kino der Toten or Five, keep using the random weapon box until you get a ballistic knife.

As soon as you get it, run to the bowie knife and buy it. You should now have a bowie knife in your left hand.

Then as soon as you have 5000 points, upgrade the ballistic knife.

Not only does the Krause Refibrilator let you revive teamates, but it's also a 1-hit kill to round 13, 2-hit kill to round 21 and a 3-hit kill to about round 29.

This is one of the best weapons due to its high power and reviving capability.