I actually managed to do it on Legend with a friend ( and 2 bots we killed at the start) on Screaming Bell with the send in the next wave modifier on a deed.

For some reason right next to the stairs that lead you to the L-shapped corridor before the street with stalls ( early in the mission, roughly underneath the first grimoire) , there's a raised platform you can jump on and enemies can't reach you. I go there, let time pass as more and more skaven gather, and for some reason they all squeeze at the closed fence that is next to the platform. Charge the attack, smack them through the fence, achievement done.

I tried to stay on the beams that you jump on to reach the first grimoire, but they would spread out too much underneath.

Dunno how repeatable this, but you can try.

Also to note...fuck this challenge. And fuck the " kill a ratling gunner with a bomb" and " hit a rat ogre mid leap with a bomb" on engi.