Poison Control: Cure poison on 5 soldiers in a single mission. Single player only.
Despite the fact that the achievement makes it sound like it has to be 5 different soldiers, it in fact can be the same soldier. You need medikits to cure poison, and due to the medikit making the holder immune to poison you have to either have 5 soldiers with medikits and just have 1 soldier get poisoned over and over, or have a support class with the sergeant 3 medikits ability (or two supports to make it even easier).
The easiest way to get poisoned is via thinmen, and on the escort missions as it seems they are the only alien that spawns for those missions even very late in the game. In my opinion its much easier to just kill a thinman and run through the poison, cure it, and then use the second action to run through it again. The poison cloud from their death only lasts 1 turn so you have to be quick.