A level 100 Shuckle can potentially deal the most damage in one single attack through the use of numerous stat boosters; by receiving the effects of Helping Hand from two allies in a triple battle, holding a Metronome, Power Trick, a Skill Swap to Pure Power, a conversion to Ice, and 6 stages of positive Attack stat changes. Also, both of Shuckle's partners must have the Ability Flower Gift and the weather must be sunny. On the 5th turn of using a Defense Curl-boosted Ice Ball (learned via Mimic) consecutively without any misses, if used against a level 1 Gible with minimum Defense stats, that has 6 stages of negative Defense stat changes (such as from being subject to Screech 3 times), and being under the effect of Forest's Curse it can deal 721,899,685 damage with a critical hit.
Under the same circumstances lugia would be as much of a goner as gible
