Occasionally in missions you will encounter Stormvermin patrols. These patrols consist of approximately 8 Stormvermind. Armoured Skaven that take quite a few more hits to kill than standard Skaven. The first time you encounter one is during the Horn of Magnus mission very early on. They can be avoided but you will most likely alert them when heading over the first bridge. Simply take part in killing all of the Stormvermin and the achievement will pop.