apparently, a level 100 Shuckle can potentially deal the most damage in one single attack through the use of numerous stat boosters, by using Helping Hand, Metronome held item, Power Trick, a Skill Swap to Pure Power or Huge Power, and the Defense Curl/Rollout combo. Also, Shuckle's partner must have the ability Flower Gift and the weather must be sunny. On the 5th impact of Rollout, if used against a level 1 Ledyba or Yanma that have been hit with negative Defense modifiers (such as Screech), it can deal 213,896,052 damage with a critical hit. :O