Althought the title might make it stage specific, there is actually a variety of AoE/area damaging skills that can destroy objects that were not meant to be destroyed, including NPC's, containers on Numa, flour bags on Slouch-O and others.
Focusing on topic now, to damage the vortex, the type of AoE we are going to use is produced by only 2 in-game skills. First one is Sympathetic Anguish on Operatives. Second is Pain, or more accurately the explosion effect of it after 3 seconds, on Engineers.
To perform this one, you just need to get rid of all surrounding mobs but one. For this purpose, I suggest you use the purple "dog" scorn unit. Just hit it with either(or both) of the skills after you bring it close to a vortex.
Although vortexes are not in the enemy count, they act as enemy units since they will not respawn and they even have a chance to drop items off them(so far only purples and pinks, might be luck though). Destroying them helps your k/d rate too, since you're free of the danger of being pushed in one or stunned close to an incoming vortex.
Lastly, this might take a while if you're doing it solo. It's more for the fun of it.
There.
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