Always have a backup browser, my game wouldn't move on chrome but when i started firefox it worked fine. Also, especially in the beginning, only shoot for triple or double shots, remember that there is no time limit. Doing this I even achieved 6 shots and plenty of quad kills and quintiple kills. And finally, I usually achieved this by staying in the corners and shooting but at level 19 stay away from the corner, I had 1200 ammo and was doing great, then a circle came from the corner and instantly killed me.
I used the magnum, then the first machine gun, then the next magnum. Got the overkill boost, and then decided to get the laser cannon just for fun. Once I got that the exp I got per level went from about 3500-57k. That next level I had every gun and upgraded the base several times. After that I kept upgrading the base and I could leave the computer for half an hour and not die cause of all the health. Once you get the laser Cannon the game is over.
 "BoomsticK"){kind=icon}
 "The Endless Zombie Rampage"){kind=icon}