i don't imagine that a lone Torterra UL with an expert belt is a huge issue.
First of all, it takes benched pokemon such as Spiritomb, Claydol and so forth to set that up, all increasing mass attack's damage. don't ko thesse set-up pokemon, leave them in play for the +10, and just bright look turtwigs/grotles/torterras and hit them before they get set up. Even if they do set one up you'll probably be hitting for 90/100, which eventually will ko a torterra, even with the constant healing. If SOMEHOW they have a lone torterra lv x with an expert belt in play, just crobat g's toxic with grass and a multi. they won't be able to switch out without playing down another benched pooemon that could easily be ohko'd, so they have to let the Toxic Fang damage rack up every turn. This, along with the 70 from mass attack, will eventually overpower a lone Torterra. It also happens to shut off Forest Murmurs.