Sacred Citadel

Ok, so this is preatty easy. Go to LOCAL GAME (to make sure you won't have any lag problems because of bad connectivity with a friend), select you're prefered hero, select ACT 3, last mission and just don't step on those acid puddles during the elevator ride.
I've also added a video showing where i stand even though i don't think that this achievement will be much of a problem

At the end of the elevator section, right before the boss fight starts, you should have you're achievement unlocked
CHEERS

While riding the elevator up, there will be two windows that the boss will poke his head through and spew acid over the floor.

Several spots on the floor will light up and poison you if you move into them. You have a brief second after they initially appear to move out of the zone and still be safe for this to unlock.

I did notice on a few playthroughs, that the lower? (closest to the screen) part of the elevator receives less poisoned areas, so sticking down there may help.