Magearna @ Leftovers
EVs: 116 HP / 252 SpA / 4 SpD / 136 Spe
IVs: 0 Atk
- Fleur Cannon
- Ice Beam
- Heart Swap
This is a great Pokemon that I use to not just take out Geomancy Xerneas, but to instead make its momentum its own.
Magearna with this spread tanks a +2 Moonblast from Xerneas at max HP with about 37% HP left. It then can use Heart Swap to take the Geomancy boost for itself and swap over any stat debuffs it already has, like perhaps from Moonblast itself or a Sticky Web to further cripple Xerneas.
The major issue with this set is that it lacks a reliable way to OHKO Xerneas without Fleur Cannon. It's possible to swap out Thunderbolt or Ice Beam for Flash Cannon to solve this, but the former hits Primal Kyogre, Arceus-Flying/Water, Lugia, Ho-Oh and Yveltal hard and the latter hits the vast majority of Ubers and AG Pokemon with a 4x weakness to Ice, as well as being neutral to Primal Groudon, which would normally annihilate Magearna. As such, I decided against it, as hitting a wide plethora of Pokemon is more important than just hitting Xerneas and Arceus-Fairy. But Fleur Cannon at +2 and with Fairy Aura is more than enough to kill a Xerneas with no HP or SP. DEF investment. I haven't tested it on others, though.
116 HP EVs and 136 Speed EVs is just enough to give Magearna 200 Speed, allowing it to hit 400 when it steals Xerneas's boost while retaining a good amount of bulk to hopefully survive revenge killing, making it outspeed almost everything in the tier without any boosts to Speed or a Choice Scarf, save Deoxys-Attack, Deoxys-Speed, Deoxys-Normal and Pheremosa with maximum investment. A Modest Nature and max SP. ATK EVs maximizes Magearna's sweep potential with a base 394 SP. ATK.
In addition, Magearna can be used to shut down plenty of other stat-reliant sweeps if it manages to survive the first hit it takes or to swap a -2 SP. ATK debuff onto another Pokemon. However, it's best not to attempt to swap in on Pokemon when using this set, as Magearna relies on its bulk in order to tank a shot so it can begin sweeping. Stealth Rock setters are probably a must to remove Focus Sash and Multiscale, but other than that this Magearna doesn't require too much maintenance to use.
Something I haven't really accounted for is a Xerneas with Hidden Power Fire, though, since my AG runs have never let me encounter one. If it manages to kill Magearna, let me know. I'll see if I can adjust the EVs accordingly.