With the newer moves Instruct and Tar Shot, and the abilities Fluffy and Power Spot we can now go even higher than Shuckle ever did. This time it's Pikachu delivering the final blow, and it has to hit a Substitute.
(Note: This would only be possible if triple battles existed in the newest games)
Even at level 100, Pikachu's attack maxes at a measly 229. However, we can get our old friend Shuckle to help us here: After using Power Trick, Shuckle can use Power Split on Pikachu, and after switching out and repeating the process 10 times, we get to 613, which is as high as it gets. Then we apply 6 tiers of Attack modifiers (4x), Pure/Huge Power (2x), Light Ball (2x) (which is why it has to be Pikachu) and Flower Gift (1.5x) (which does not stack, so we can only do one) which leaves the attack at 14712.
The target will be a Smeargle with 7 or less Defense, which after modifiers becomes 1. It has to use Type reflect on a Genesect, and then get Forest's Curse applied for an 8x weakness to fire. Smeargle must also be under the effect of Tar Shot, which doubles damage taken from fire-type moves, and have the ability Fluffy, which also doubles fire damage, but not if the move makes contact. Since the move we are using, V-create, makes contact, we use a Substitute to avoid this. The sun, which is necessary to activate Flower gift, will also give a 1.5x boost to damage. If we use Color Change to change Pikachu's type to fire, STAB (1.5x) will occur, and with a crit (1.5x) the total damage multiplier becomes 108.
V-create has a base power of 180, and since this is a triple battle we have two teammates who can both use Helping Hand, boosting the power by 2.25x. Now Instruct lets two of the pokémon on Smeargle's team instruct those teammates to use Helping Hand again, for another 2.25x. V-create is activated using Me First, which again multiplies the power by 1.5. Lastly, since Flower Gift didn't stack, we have another spot for an ability. We fill it with Power Spot, which boosts the power of moves used by all teammates by 1.3x. The total power of our move is now 1776.9375, rounded down to 1776.
To calculate the damage, use the formula Floor((Level*Power*Attack)/(Defense*50)+2)*Modifier. Level is 42 for a level 100 pokémon, so we get Floor((42*1776*14712)/(1*50)+2)*108=2,370,378,816.