This one isn't too tough to get!
- First, you need some luck to be able to buy the "Gas Mask" Augment from one of the various shops!
- Next, you need to find a stage that has the "Acid Cloud" symbol. I was able to use the Underwater stage in Tournament 4 for this one!
- Last, make sure to equip the "Gas Mask" and then simply head over to the Acid Cloud on the map. It will be green with skulls and moving each turn. Simply spend a turn in the cloud and you'll have your achievement!
Best of Luck!