24 HP, 252 Atk, 232 Speed
Item: Life Orb
1. Dragon Dance
2. Grav Apple
4. Sucker Punch
Flapple's already good Attack stat is catapulted by its ability to use both Hustle and a LIfe Orb; With Hustle, that 110 Base Attack stat becomes the effective equivalent to a much more staggering 190 Base Attack (albeit with a chance to miss,) and with a Life Orb on top of that, it gains an even more effective attack than a Mega Mawile! In short, Flapple acts as an offensive powerhouse, utterly wrecking anything that it can get a solid move on even if doesn't set up. However, it's held back by a poor base speed stat in 70, which makes it really hard to outspeed things that aren't trying to be bulky, and even more so is that its poor defensive stats and six weaknesses compromise its ability to stay on the battlefield. Flapple has to be played with carefully in order for it to get results. And what also really doesn't help Flapple is that its Grass/Dragon STAB cannot hit Steel-types effectively, and that it doesn't learn any moves that can hit them for Supereffective damage; in fact the only move that it learns that is even neutral to Steel is the listed move Sucker Punch. Flapple also doesn't really have good selection of STAB moves; Grav Apple and Dragon Claw are weak at only 80 BP, and while Outrage is strong it locks you in for 2-3 turns. This is also all further exasperated by the fact that Hustle makes all your moves have only 80% accuracy.
Watch for chances Flapple has to come in and set up a Dragon Dance. Choice-locked Electric or Water moves, Pokemon too defensive to deal with it in one turn, or slower Pokemon that simply might want to switch out of an incoming Grass-move are all examples of this, on top of any strategies your team might already have. Once that happens, it's all but guaranteed that something is going to die. In Smogon UU and below, there aren't many bulky Steel-Types that can get in Flapple's way, and when Dynamaxing is available, Flapple utterly bulldozes through most of those, too. In fact, Dynamax fixes many of Flapple's problems: Max moves never miss, so Hustle's accuracy doesn't matter, its moves get crazy-high base power (130-140 BP) without drawbacks, and its HP doubles, so it becomes twice as Bulky. On top of that Max Darkness, while still not supereffective against Steel, has enough power behind it to match that of a STAB Grav Apple in base form, which is another way of saying that it can hit Steel-Types very hard. Flapple will basically just roll over anything that tries to get in its way with all that power it has at its disposal. Flapple can beat either Ferrothorn and Corvinight quite soundly if they switch into a Dragon Dance; The damage it takes becomes halved by Dynamax, and then it becomes reduced again by its Max Wyrmwind, since it lowers their Attack stats, and that neuters possible retaliation with moves like Gyro Ball and Brave Bird. On top of that while Flapple is slow--This moveset only outruns Pokemon with 110 Base Speed after a Dragon Dance, it more than makes up for it by having priority Sucker Punch. Most Pokemon faster than 110 Speed are fragile, and since you have +1 Attack and an Attack stat higher than Mega Mawile, it's very possible for faster Pokemon to get KO'd by Sucker Punch. Sucker Punch is also useful to revenge kill things and helps it remain useful for a team if it can't set up. I'd also like to mention that Grav Apple on the switch + Sucker Punch follow-up is a good combo, since Grav Apple has a 100% chance to lower Defense.
Also, don't bother Gigantimaxing Flapple. This switches Max-Overgrown with Gmax-Trartness, and Max Overgrown is more useful because it lets Flapple heal itself a bit and powers up every Grass-Type move it uses for the next five turns.