Real easy. Just like knocking out your 200 vanquishes with any character. Load up a solo with AI Giddy Park and go to the plants side with your Electric Slide character. I hung out by the steps that lead up to a ferris wheel you can get on. That's just a location reference. You'll know you're in the right area because you'll see targets and the plants launch area up is up above you pretty far away. Hit RB to activate this ability then press it again when you want it to become like a barely 2 second AOE effect that kills quickly. Just let some group up or kill stragglers until it sends a couple at you at once. Hit the effect (RB a second time) where they're landed and you'll have the 20 in no time. It's important to know you're hitting RB a second time to do the electric explosion thing because only hitting it once does a random tornado that doesn't kill quickly.