Get the sickle (add in Ballistic Knife for faster swipes) from across from Speed Cola.

Now, get George near the wate,r somewhere where you will have a few minutes undisturbed.

Stab him on the dry ground, this will cause him to do his longest 'aargh', immediately slip behind him and stab him the entire time that he is raging. Then run him into the water and stab him there (he rages for a shorter time) then get him back out and repeat this process.

The knife does a good amount of damage and the time taken will be much less than that taken up by running to the box, getting a weapon, firing it at G, getting another, et cetera.

Remember that you can't buy perks/weapons during the time that you have the Wunderwaffe perk (this assumes that yiou have finished the 'Ensemble Cast' achievement for MP zombies or the 'Stand In' achievement for solo zombies; otherwise you get a temporary Death Machine instead).

Killing G on round 20 was relatively quick using this method.