In my experience, zombies are easy to kill except for when they use a smoke bomb to break your teamwork and attack. Chuck and Helga is only 3 hits in sniper and Tyrant is about 8 - 10 critical hits. But if you are with your teammates, zombies are easy to kill as follows. Chuck and Helga is only 1-2 hits and Tyrant is about 4-5 hits.