Ended up 3rd overall, 2nd in Canada. Here's my team and the logic behind it:
Talonflame @ Choice Band
Adamant, Gale Wings
4HP/ 252Attk/ 252 Sp
Brave Bird
Flare Blitz
U-turn
Sleep Talk
Everyone already knows Talonflame is a good pokemon. Most battles was just trying to get everything down to a manageable HP so that talonflame could clean up. And one piece of advice that seemed to win me a ton of battles - use brave bird on M-Mawile, as it will try to sucker punch you every time and can't hit you otherwise. And a choice banded BB still does ~25%, so its good for revenge killing it if its around half.
Yveltal
Life Orb
Lonely, Dark Aura
80Atk/ 172 SpA/ 252Sp
Oblivion Wing
Dark Pulse
Sucker Punch
Taunt
This was not the nature or spread I wanted but its the only yveltal I had, and let me tell ya, this was actually my mvp. Led off most battles and got around 90% usage from me. Could take any one hit, then got most of its hp back with a life orb OW. Plus it OKHO'd any Aegislash I faced with a dark pulse, or made sure to leave a hefty dent before it died with a sucker punch. I had no idea how valuable this guy would be, but he came through.
Xerneas @ Power Herb
Timid, Fairy Aura
80HP/ 172 SpA/ 252 Speed
Moonblast
Tbolt
Psyshock
Geomancy
Not much to explain here. With a chance to set up, it was sweeping time. I was fortunate to get a Timid Xerneas with max speed IVs, so I maxed them out to outspeed other xerneas and get the jump on them. I went with psyshock, which was weirdly not used often, for pokemon like M venasaur.
Rotom H @ Sitrus Berry
Modest, Levitate
252 HP, 200 Def, 48 SpA, 4 Spe
Overheat
Volt Switch
WoW
Light Screen
Way underused IMO. He deals nicely with Xerneas and Yveltal, counters M-Mawile, Scizor, Talonflame and (most) Charzaird Y, does work on Aegislash. Just a great choice in this specific tournament.
Mawile @ Mega
Adamant, Intimidate
252 HP/ 180 Atk/ 60 SpD/ 16 Spe
Play Rough
Iron Head
Sucker Punch
Sub
The legendary Counter. The extra Sp.D investment was to take hits from the legendaries and could OKHO them with a stab or super effective move. Intimidate forces lots of switches which makes Sub a boon for ease of prediction (and forcing rotoms into attack moves). The 16 speed to outspeed other mawile, naturally.
Scizor
Focus sash
Adament, Technician
252 HP/ 252 Atk/ 4 Spe
U-turn
BP
Aerial Ace
Sleep Talk
This guy was deadweight and only saw about 5% usage from me. I originally wanted a mewtwo counter with pursuit, with the sash being a safeguard for flamethrower and I also wanted him for xerneas. Yet I also wanted ace/sleeptalk for the many sleep inducers. In the end, the only role he had was killing the (too many) vivillons and other sleep inducers. Not bad, but I think I would have been better served with a mewtwo with sub, a pokemon that i really wanted by the end.
My team was quite weak to M-Kang as it could fake-out and return/EQ KO my whole team, so Kang was always a team effort. I also lost to a few more gimmicky, luck-based strategies which is fine in a tournament of 80 games. I also got swept, 3-0 by an assault vest ryperior who set up with rock polish at one point, haha!
Three notes:
1) I don't understand people complaining about legendary pokemon. That was sort of the point of this tournament. If you don't want to see them, participate in another tournament! I for one relished the opportunity to finally use the guys that have been shelved all year, and I see nothing unfair in that because everyone had the choice to use them also.
2) I also found it funny how hard it was to gain points at the top of the ladder. I was at 1940 at one point, and lost to a guy with 1850 and lost 40 (!) points. I won five in a row to get back to 1940, then lost again and went back down to 1900. I also battled a guy with a 1450 rating and gained a total of 1 rating point. Why would they even make us battle, it was not a benefit to either player! Just found that funny.
3) Does anyone know if winners benefit from these types of tournaments? I did this for fun, but was curious if we got anything for placing, as I have not seen anything.
Anyways, thanks for reading. I had a lot of fun battling for 3 hours straight into the middle of the night for a couple days. I had a lot of good games against people here.