I just barely won against pic related, only because his rain-boosted Hydro Pump missed just as we were down to our last Pokémon and my Light Screen had just worn off.

His team is fantastic. Manectric and Rhydon both have Lightningrod, so you cannot target Crobat, Starmie, or Pelipper with Electric moves. Pelipper is thus free to set up Rain Dance, guaranteeing hits on Manectric's and Starmie's Thunder, and boosting its own Surf and Starmie's Hydro Pump to hit overwhelmingly hard.

Add to that that your own team most likely sucks because of the limited selection of available Pokémon and moves, and this is gets close to Ghetsis levels of difficulty.