I can confirm these achievements can be done on a local game using the unlimited ammo (and I SUPPOSE any other will work too) add-on from the workshop, that makes it very easy to do:
Just pick a spot in any official map where zombies come fromonly one place, and start throwing molotovs and shooting there, you'll never run out of ammo or molotovs so you should survive forever. When a tank is coming, just burn him and then focus on him. He should die in no time.