EDIT #705: The highest damage output, with legit movesets and abilities, without counting Rollout (where the highest damage output would belong to Shuckle), is a Darmanitan holding a Metronome on the 10th turn of using Flare Blitz with an Adamant nature, max Atk, +6 Atk boosts, in the Sun against a lvl 1 Paras with the Dry Skin ability with a -Def nature, -6 Def boosts, a 0 IV in def, with a Helping Hand boost, a Me First boost, and a Critical hit. This yeilds a maximum damage output of 22,039,884 damage.
Of course, this is nowhere near the maximum damage output Shuckle can give, at an absolutely jaw-dropping 360,949,532.
EDIT # 1,000: Can someone check Stored Power? I really need to stop running dmage calcs and get to work >.>
And there is no Huge Power involved in what I said before O.o