I'm currently 20-10. I'm going to make a little summary of what works on my current team and what doesn't.
Works:
Groudon. This is fairly self-explanatory. Groudon is one of the best Pokemon in this format and can quickly turn a game around. I have beaten Kyogre and Rayquaza together with my Groudon and Ferrothorn. Basically Attack-Protect and Protect-Attack. By the time they figure out what you're doing, they will be almost dead. I am running Modest Groudon, with Eruption, Earth Power, Fire Blast, and Protect. It is fast enough to outspeed Smeargle and can survive opposing Earth Powers from Groudon. Which is super important.
Yveltal. I use Pyritie's bulky set and it is just perfect. I am considering moving my 4 EVs from Attack (Sucker Punch and Foul Play) to Special Attack (Snarl). Let me tell you, Snarl is OP. Faced with Kyogre? Snarl so Ice Beam only does like 25%. In fact, Snarl in the face of any special attacker. It feels so good to send Groudon in on a crippled Kyogre.
Kangaskhan. Kangaskhan is majorly feared, and it really should be. In fact, I think I only used my Mega Mawile once, and it was in one of the first battles. I'll get to Mawile later. First of all, Inner Focus was the best decision I've ever made. With this ability, opposing Kangaskhan and Liepard don't scare me at all. Just don't Mega-Evolve until the next turn. Double-Edge doesn't even have to be explained. I OHKO'd a Primal Kyogre with it once! It was especially frail, though. I considered dropping Low Kick for Power-Up Punch, but it was useful against Dialga and opposing Kangaskhan. Sucker Punch wasn't used super often but it came in handy sometimes.
Crobat. When faced with Crobat versus Talonflame whole team building, I finally picked Crobat and went with it. And I am SOOO glad I did. First of all, Quick Guard was dropped for Haze. If you think this is a dumb idea, Haze comes in handy more than Quick Guard. And if your other leading partner is Kangaskhan like mine usually was, Inner Focus is perfect. During a Crobat-Kang versus Smeargle-Xerneas lead, I taunted Smeargle and faked out Xerneas. I don't exactly remember the rest, but Xerneas got up a Geomancy. I switched into PDon, ate up a Dazzling Gleam, then Hazed Smeargle's boosts and Geomancy away. I won that game.
Kinda Works.
Ferrothorn. Ferrothorn is not good in almost every situation like Kangaskhan or Groudon, but it can excel if the conditions are right. I mentioned somewhere about beating RayOgre with it. I'm keeping Ferrothorn on my team, at least for now.
Didn't Work.
Mawile.
This disappointed me because it was the one I wanted to use the most. Long story short, there was no situation in which I wanted to use it rather than Kangaskhan as the mega. And if it's not the mega, it's dead weight. A Salamence and Xerneas check, along with Intimidate and Huge Power, seems really good on paper, but it just didn't work out on my team.
My hands are tired now.