That's my girl. But those Thunders did... A lot :( And that Rotom's a tough bugger, I'd expect being slammed into a window like that to do damage... Guess that's what makes it a crit.
Alright, go for another Poison Tail, then Crunch, before Poison Tailing again. But if we manage to poison him the first time, use Venoshock instead. If he uses Will-o-Wisp on action one or two, evade it with Dig, and Disable it. If it happens on action three, just Dig to evade.