Achieving it is not that difficult. There is one type of zombie here, a hazmat with a yellow jumpsuit. You have to shoot him, then for a short period of time (about 5-10 seconds) a green cloud with poisonous gas is released from it. You have to enter the gas and wait for your character to animate on the screen that you are taking damage and green-yellow spots. You will then have 1% progress to be achieved and 99 more times you will do so. The achievement will be unlocked.
It took me about 5 hours. Most often, this zombie spawns on an easy or normal level
Edition - 2022.08
Unfortunately, after the September update of 2021. when this DLC was added, new types of zombies were added. unfortunately, due to this update, the zombie hazmat will spawn less frequently.