Jibaku pretty much recommended all of the things I was thinking about recommending to deal with Xerneas and Pyritie took Cresselia, but I'll add another suggestion for dealing with Xern that hasn't been brought up yet I guess.
Mawile-Mega @ Mawilite
Ability: Intimidate
Level: 50
EVs: 252 HP / 148 Atk / 108 Spe
Adamant Nature
- Iron Head
- Play Rough
- Sucker Punch
- Protect
EVs are up for debate depending on what the rest of the team ends up being, keep in mind. The Speed EVs allow it to hit 84 Speed which is an amazing Speed stat as it outspeeds +2 max speed Xerneas after a Thunder Wave and underspeeds the minimum Speed Primals under Trick Room. A lot of people put 252 EVs into Attack over HP but every time I do that with Mawile I end up missing the small amount of bulk I lose. It threatens not only Xerneas but opposing Yveltal really well and synergizes with our team's Yveltal because Mawile's Sucker Punch is boosted by Yveltal's Dark Aura. It can be fairly tough to use since it struggles against both Primal Pokemon though I was able to use this set to get top 16 at Madison Regionals this year (though it was on a Dual Primals team and not a Yveltal one).
Edit: Apparently we also have to recommend the Groudon/Yveltal sets along with the 3rd mon. Here's something I guess.
Ketchup (Groudon-Primal) @ Red Orb
Ability: Desolate Land
Level: 50
EVs: 60 HP / 188 Atk / 4 Def / 4 SpD / 252 Spe
Jolly Nature
- Precipice Blades
- Fire Punch
- Substitute
- Protect
The EVs allow it to take 3 hits from a neutral max SpA Mega Salamence Hyper Voice with spread damage. Maximum Speed EVs are to hit other Primals before they can hit you provided they aren't also max Speed (though you'll be in a speed tie situation bar no Speed control has occurred). The rest of the EVs were put into Attack for damage. Precipice Blades and Fire Punch are strong STAB moves that hits most things pretty hard; A lot of things could be run over Substitute (I'm mainly thinking of Swords Dance) but I decided to put it on here to help keep it up and running more in the face of a Primal Kyogre since it'll be able to put up a Substitute before it gets hit by a Water move (keep in mind that this Primal Groudon is max Speed and most Primal Kyogre don't tend to go this fast).
Yveltal @ Life Orb
Ability: Dark Aura
Shiny: Yes
EVs: 244 HP / 4 Atk / 28 Def / 4 SpD / 228 Spe
Jolly Nature
- Foul Play
- Sucker Punch
- Snarl
- Protect
The spread and moves for the Yveltal I took from a friend of mine who often uses Yveltal at VGC events (I don't think he has a Smogon account) with the exception of my putting Life Orb over Black Glasses. He said that the Speed EVs are to outspeed base 95 Pokemon like Kyurem-White and non-Mega Rayquaza with the remaining EVs put into bulk. According to him it "lives everything except for a LO Raichu Volt Tackle, a non-Intimidated Mega Mawile Play Rough and a Moonblast from Xerneas." He used this set to get t64 at US Nationals this year; here's his
team report.
