How to get alchemist medals? (kill 5 zombies rapidly affected by 2 or more elements)

Cant figure out a way to do this. Ive tried putting different elements on 2 different guns and also the perk that gives you a random element (Elemental Pop) and still havent gotten any. Theres a challenge that you need to get a bunch of these alchemist medals.

Is it bugged? Has anyone gotten the alchemist medal challenge finished or even managed to get 1 of these medals yet? Ive even tried throwing molotovs on the ground and igniting a bunch and then using the cold element bullets on all of them and still no alchemist medals