You MUST do it on solo, because the more players, the more health he gets. A strategy I got for is:

Step 1) Either Keep restarting the game until the box spawns near you, OR you can go find the box after step 4

Step 2) Open the doors to the power, then turn it on. OR if the box has spawned near you like infront of the lighthouse or near the power room, open those doors/ use that first.

Step 3) Get to Juggernog! Presuming you have already bought a Quick Revive, You will need Juggernog to hold onto your current Quick Revive for a little longer, and to protect you from those double swiping zombies bastards.

Step 4) Go Find the box if you haven't already, but if you have, save up your points and keep going on it until you get a Ray Gun AND a good gun like RPK,HK21,Scavenger something along those lines, just NOT the V-R11.

Step 5) Next round, make a crawler. Then use ALL of your Ray Guns ammo. Don't try to worry about ammo, but be careful not to kill the crawler as if you do, a new round will start and you may not have enough ammo to survive or you may not have enough to kill Geogre afterwards.

Step 6) USe the Rest of your other guns ammo and more if necessary. I burned through an entire Ray Gun, An Entire RPK and half a Commando (Used the box and swapped it for the RPK) before I killed him, and remeber when The blue light on George flickers Orange, He's about halfway dead, and when it stays Orange He is about to die.

Have fun killing Geogre and getting that trophy!