What about Zombies that will need to kill just from Rifles (Rifles Wave). Example 10 players in game will come up only 20 zombies and each will need 20 head shoots so one player can shoot 1 zombie only 2x in head i dont know but will be hard coz some players have no popints at all to find rifles in box 
so will need spend coins in Shop heheh and each zombie will give same bonus like Boss but little less i dont know Fabio but will be good wave that i think if you like i will explain you that my idea more close on TS so you can understand it bit more i know my writing is not the best hehehehhe
so will need spend coins in Shop heheh and each zombie will give same bonus like Boss but little less i dont know Fabio but will be good wave that i think if you like i will explain you that my idea more close on TS so you can understand it bit more i know my writing is not the best hehehehhe