well here's a stratagy me and 4 of my buddies are trying ATM(or at least 2 of em are) 4 of us started a game on Shan-gri-La acquired 2 focusing stones doing the easter egg twice and then 2 of us that didn't get the stones backed out sso the other 2 could try for high rounds with less zombies out. I'll have to wait till tomorrow to see how far they made it, it was cool that we were able to get 2 playerss with stones by round 13 so that's a stratagy to try if anyone wants