it sucks when you upgrade everything then you have 0 stars then you get 850 but then you get 1000 in the first 5 secs and you have to wait for a minute to be finished like a waste of time
i got it nevermind: this is how you get the 10 kill streak badge:
1: complete level 40 then it will give you the mini game
2:get 10 kills before a zombie hits the ground
3: wait for the badge and dont get a better killstreak or else the time will reset