Since Toxtricity are electric/poison type, and have therefore a quadruple weakness to ground, a Poke of that type which is espeacially slicey I would say. Maybe a Garchomp? :3 They look kinda frightening even.
Maybe Diesel made to much noise for the Garchomp and his group of Gible and Gabite or so and instead of being polite he just pissed them more off and wanted to fight them? I'm not good in coming up with stories. ^^

Funny you mention that ;) After drawing this I made an antagonist for them.....

A garchomp antagonist? Well, this poke certainly fits the villain role. :)

Not a Garchomp, but another ground type!

A Geodude that knows self-destruct (I say this because one of the few shinies I've ever encountered was a Geodude, that knew self-destruct.....)

Oh rip! My boyfriend found a shiny Graveler that also knew self-destruct lol

Trouble THATS what