Dude, it's Rollout, Miltank's most infamous move. The move that can and will OHKO anything after a few turns even though it has "only" 30 Power. I meant painful in a good way. Can we give a preemptive moment of silence for Nuzlockers who pickWhy would it be painful though. If it's just cause the move isnt exactly great, well, consider we talking of a Rock gym and there arent really many low level Rock moves (plus Rock has the problem that tends to trash on early game pokemon, aka bug and flying types, and it's not ideal to have strong stabs for it in the very first gym).
I'd see it in opposite way, and think it's a *good* sign since they actually had a interesting thought with it (the move gets stronger with use but also locks the user into it, letting you swap in a resist and then counterattack)
Edit: I could have sworn I typed Chimchar. I was even thinking Chimchar while I typed it. Well, anyone that pick Charmander deserves to lose for cheating. No moment of silence for you. :P Seriously though, Rollout made Shuckle into a offensive powerhouse in Jrose11's GS Shuckle Solo. He didn't bother with Toxic because Rollout was that devastating!