Don't have much time but I'll try and run through this quickly for ya
I'm going to go ahead and assume that you don't know what tiers are, so I first recommend you take a quick look over this:
http://perfectpokemon.com/threads/official-pokemon-oras-tier-list.7138/
This is basically our official tier list. We differ to Smogon in a number of aspects (and I'd generally recommend going with their playstyle for the most part) but I'll talk you through the Perfect Pokemon metagame
The main tier is OU (Overused) and you can use any Pokemon/Items in this tier or below: any Pokemon/Items in UU, RU, NU, PU, NFE, LC are all usable. Although people don't align with my opinion 100% on this one, although they think he is probably top 3 of OU, I personally believe that Greninja is the biggest threat in the Perfect Pokemon OU metagame, with the possible exception of Mega Mawile. This is due to the Protean ability + Life Orb + incredible speed tier + an amazing movepool making Greninja an extremely unpredictable sweeper, cleaner, scout and basically anything that doesn't involve taking hits. The set you'd probably want to run would be something like:
Greninja @ Life Orb
Ability: Protean
EVs: 40 Atk / 216 SpA / 252 Spe
Naive Nature
- Ice Beam
- Dark Pulse
- Gunk Shot
- Low Kick
However, do note that Greninja is not the best thing to
build around. Greninja's moveset is highly dependant on what other Pokemon you have on your team and what they lose to. If you're looking to win with Greninja, you have to eliminate a few common things: anything faster than it, Chansey and Porygon2. To deal with the two latter Pokemon, I'd recommend a Pokemon with good fighting type coverage such as Mega Pinsir, or if you don't want a Pokemon that's 4x weak to Stealth Rock then either Mega Lopunny or Mega Medicham.
Medicham @ Medichamite
Ability: Pure Power
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- High Jump Kick
- Ice Punch
- Fake Out
- Zen Headbutt
OR
Lopunny @ Lopunnite
Ability: Limber
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- High Jump Kick
- Return
- Fake Out
- Power-Up Punch
In this case, I'd actually probably recommend Lopunny as you're looking for something fast to take out the threats that outpace Greninja, but Medicham also works.
Now you currently have two hyper offensive threats, I can kinda tell that you're going for a more offensive team, but I'd probably like to see a Fire/Water/Grass core + Lopunny + [Wall] + [Support] on this team probably. As for a fire type, I'd recommend defensive Heatran as it's able to take on Talonflame which you currently struggle with, or if you're feeling ballsy, possibly Volcanion, although you don't have a fantastic matchup against Talonflame in that circumstance. As for a grass type, I'd love to see how Tangrowth works on this team as it could honestly be super trolly, provide you with bulk and support, etc.
Tangrowth @ Leftovers
Ability: Regenerator
EVs: 244 HP / 216 Def / 48 SpD
Relaxed Nature
- Sleep Powder
- Giga Drain
- Knock Off
- Hidden Power Fire
Heatran @ Leftovers
Ability: Flash Fire
EVs: 252 HP / 188 SpD / 68 Spe
Calm Nature
- Lava Plume
- Toxic
- Taunt / Roar
- Stealth Rock
This team is looking really solid now. You have bulky Pokemon in Heatran and Tangrowth to deal with sweepers in general, you have Stealth Rocks to use as entry hazard, you have support in terms of Sleep Powder, a win condition in the form of Greninja and possibly Power-up-Punch Mega Lopunny if you can get that right, etc.
Honestly not sure where to go next, but I'm trying to use
http://sweepercalc.com/rmt/ and
http://hidden50.github.io/brmt/ to work out what would be best for your team.
Apparently this is the list of threats that you currently lose quite badly to. I'd recommend plugging in sets of Pokemon in the last two slots and trying to find ways of eliminating Pokemon down from this list. Talonflame + Rotom-W manages to cut it down quite well but I'm honestly not amazing at doing this.
Keep me posted on this
I'd love to work on this team with you
EDIT: Air Balloon Heatran + Garchomp + Clefable does wonders. Only loses to Alakazam, Mega Medicham (even though Rocky Helmet + Rough Skin Chomp mostly beats non-Ice Punch variants) and Mamoswine (preserve AB Heatran)
EDIT 2: Tornadus-T + Ferrothorn is good
EDIT 3: Clefable + Landorus-T is a little worse but works, and Scarf Lando-T helps eliminate a number of remaining threats
EDIT 4: Kyurem-B + Cresselia strangely works