Hey, I decided I'd take a look at your team. I like to separate my, assessments, into 3 sections. Without further ado here we go.
Typing: Well, the typing is pretty good. You have a x2 fairy weakness and a x2 grass weakness. These aren't that big of a deal and I wouldn't worry about it enough to change any of the pokemon on your team, just keep in mind to watch out for those 2 types.
Synergy: Gardevoir isn't a lead. You have a decent mix of different roles, but some of the pokemon don't work too well in their roles, but for the most part your synergy is ok.
Movesets: This is one of the hardest areas to learn, that's why smogon has this. But I'll go through each individual pokemon and give suggestions.
Gardevoir: The moveset is fine, but the EV's are a bit odd, especially for a lead. You need max speed on MGardevoir and still it isn't the best lead (generally leads are scouts, they are fast and have volt switch or U-turn).
Rotom-W: Don't use a rindo berry on rotom-w, just don't. Give it scarf for a lead (but you have plenty of leads/scouts/pivots) or lefties for a wall. It's moveset is fine.
Sylveon: The most basic sylveon there is. It's what most people expect when they see a sylveon, so keep that in mind. If you want sylveon to be a cleric than this is a good set.
Greninja: Great moveset on greninja, but their is one huge problem. It absolutely needs max speed. Every greninja needs max speed. I can't stress that enough. Have it's nature be up in speed and down in Defense (hasty iirc). Give it 44 att evs if it has gunk shot.
Landorus-T: I prefer Knock Off to superpower, but that's all on Lando.
Lucario: No air balloon. Air balloon is pretty much only used on heatran (for the most part). Give it a life orb and switch hi jump kick to close combat. Give it max speed EVs.
Well, the team is pretty good, but your EV's and items need some work. I like this team, but it needs a few changes. If you have any questions please ask in comments. Hope this helps!