I think it is right to give this deck another chance because of the new Herdier from Sun and Moon giving extra consistency.
Pokemon: 14
3 Palkia-EX PLS
2 Deoxys-EX PLF
3 Lillipup BW
3 Herdier SM
3 Stoutland BCR
Supporters: 12
4 Lillie
3 N
2 Lysandre
1 Xerosic
1 Pokemon Ranger
Items: 21
4 VS...
Magearna doesn't seem too important since ideally you will have Solgaleo with 0 energy attached by your opponent's turn. But Metal Fortress Bronzong looks nice.
-1 Magearna-EX (STS)
+1 Bronzong (FCO)
There is not much to do against Volcanion but accept the loss it appears, unfortunately...
One strong energy brings Crunch to 50 (a 4HKO) instead of 40 (a 6HKO) so you can do enough damage to knock things out before they (or you, for that matter) use a switching effect to deny the KO.
Edit: In to 50 doesn't make sense :P
I fixed it.
Dropping Kukui makes it harder for Yveltal and Umbreon to OHKO shaymins. (90+20=110), but (90+0=90) :(
I would say keep that in the deck, if only for that purpose.
Also, make room for Lysandre.
-2 Red Card
+2 Lysandre
Red Card isn't necessary, while Lysandre is amazing in every deck.
In my (Expanded) league, I've seen 1 Volcanion and 1 Pyroar player since the cards came out, but my league has very unhealthy levels of Greninja, so I can see its inclusion or disinclusion dependent on the local metagame.
There are two Mega Gardevoirs so for the moment I will assume you are using the brilliant arrow one.
To make this standard, @Jedidr:
-2 Spritzee
-2 Aromatisse
-2 Super Scoop Up
+3 Fairy Drop
+1 Sycamore
+2 Xerneas BREAK
Fairy drop is nice for making Gardevoir stick around longer, preserving...
Oops, I just realized Lucario needs steel energy so you will never be able to attack with it.
If you prioritize Lucario:
-4 Fighting Energy
+4 Steel Energy
If you don't:
-3 Riolu
-3 Lucario
+6 ?
A good thing to replace Lucario for would probably be more consistency cards, like Oranguru from...
Ok, now that that swap has been made there are only 4 draw supporters in the whole deck.
-2 Pokemon Center Lady
-2 Shauna
-1 Repel
+5* Lillie/Sycamore/Birch
Shauna should be replaced for Birch if you like the shuffle in effect: but this will increase your draw supporters to 7, ideally though...
My least favorite would have to be Roaring Skies. That set introduced Mega Ray (colorless), Shaymin, and a VS Seeker reprint:
Yay cards I "need" but can't afford!
EDIT: I would argue that Evolutions is a great set, but this is from a biased Gardevoir player :P