The first thing you'll need to learn about competitive battling is EVs and IVs (to a lesser extent). I'd explain EVs, but I'm terrible at explaining them for some reason. I think
this page will explain it to you well enough. Items, natures, and abilities are important too. You don't need to worry about tiers for Wifi battles on your X/Y game; all Pokémon are allowed in Wifi battles. However, you can't have 2 of the same Pokémon or item on a team. Tiers are more for official tournaments and competitive battling places like Pokémon Showdown.
I'd give you EV spreads and movesets for your current team, but its a huge pain in the ass to create an entire team for someone, especially when most Pokémon can run more than one set and everyone has their own preferences and styles. What you'll wanna do is visit Smogon, the premier competitive battling info site for sets for your Pokémon. Keep in mind that Smogon has yet to officially post sets for the Gen 6 Pokémon. Use your own judgment to alter Smogon's Gen 5 sets for the Gen 6 metagame.
Flygon:
https://www.smogon.com/bw/pokemon/flygon
Absol:
https://www.smogon.com/bw/pokemon/absol
Lucario:
https://www.smogon.com/bw/pokemon/lucario
Yveltal:
https://www.smogon.com/forums/threads/yveltal.3490005/
Noivern:
https://www.smogon.com/forums/threads/noivern.3490018/
The links for Noivern and Yveltal are from Smogon's forums discussing
possible sets for the Pokémon. Sylveon I'll give you a set for, so you know what info to give when discussing teams, and because I don't see it doing much else than this:
Sylveon w/Leftovers
EVs: 252 Special Attack, 252 HP/252 Sp.Def, 4 Speed
Moves: Calm Mind, MoonBlast, Shadow Ball, Psyshock
Nature: Modest/Calm
Ability: Cute Charm (it doesn't have much use for Pixelate)
Although, when the PokeBank is available, it could do this:
Sylveon w/Lum Berry
EVs: 252 HP, 252 Sp.Def, 4 Sp.Attack
Moves: Wish, Protect, Moon Blast, Shadow Ball/Psyshock/Toxic
Nature: Calm/Modest
Ability: Cute Charm
So do some research, then come back here with somewhat more specific questions and your team ideas and sets. Hope this helps.