This sounds like a lot, but it is not in Dead Rising 4. Just wait a bit after first two cases and you get to drive outside. Just Ram in to huge crowds of zombies, rinse and repeat, if you want to hurry this achievement (and to get the other ones requiring to kill bigger numbers).
There is also a nice section in case 3 where you get exosuit with ice power up and there is an ocean of zombies waiting for you. Just reload the checkpoint when you run out fo zombies to kill. And turn your sound down, the ice powered exosuit makes hell of a racket.