Choice Band isn't even around yet, so running Jolteon is kind of pointless, at least for the time being. Also, Wide Lens is a waste of space, especially if you get rid of Jolteon.
-1 Jolteon
-3 Wide Lens
I can see Yveltal BREAK being a little useful, especially if you're spreading damage around with Umbreon, but running 2 is a little overkill.
-1 Yveltal BREAK
Not only is Timer Ball unreliable, you only run a few evolution cards, most of which are eeveelutions, so you can just use Energy Evolution to get them out. The last evolution is Yveltal BREAK, which, as I said, isn't that crucial, so it's not really worth the Timer Ball spaces.
-2 Timer Ball
Maxing out the DCE count is a must, especially considering that all of your attackers can make use of it.
+1 Double Colorless Energy
Stadiums are important. I'd run a few to win the stadium war, if nothing else.
+2 Parallel City
Other than Olympia, you don't really have a solid way of retreating.
+2 Float Stone
+1 Escape Rope
Hand disruption is good, and Delinquent is definitely helpful if you only decide to run a couple stadiums.
+1 Delinquent
Here are a few other suggestions:
- Although Yveltal BREAK can take advantage of Umbreon-GX and its pre-evolution's attacks, I'd do some testing to see how helpful it really is.
- Even though it's a nice consistency booster, I don't really think that the third N is that useful. This deck tries to be aggressive and get an early attack off, so N will quickly become unuseful.
- I think Professor Kukui could be nice. It would bump up Umbreon-GX's Shadow Bullet up to 110 to get the OHKO on Shaymins.
- Team Skull Grunt could also be fun with the disruption theme. Having hand and board disruption is always powerful.
- Choice Band and Alter of the Moone from SM Strengthening will be nice in this deck. Dealing 30 extra to GXs and EXs would make the math a bit nicer on Umbreon, and Yveltal as well. Umbreon's Shadow Bullet and Yveltal's Y-Cyclone would OHKO Shaymins. Umbreon would then 2HKO everything, and an Evil Ball with 3 energy on both active 'mons would do 170. Alter of the Moone could give free retreat to anything in the deck while they have Dark Energy on them, which not only would increase maneuverability and options when attacking, but would also allow you to take out some other switching cards, and free up space for other cards you might want.
- Maybe try and see if you can find a place for Professor's Letter. It lets you get out basic energy to evolve into your Umbreon.
Final Notes:
- I don't think you should try and fit in a maxed count of Max Elixir or Trainers' Mail. Ideally, you want to be manually attaching a fair amount to quickly evolve your Eevees, so having extra Max Elixir could pull energy from your deck that you could be drawing into. For Trainers' Mail, 3 is plenty to draw into it often, and if you have 4, you have an increased chance of T-Mailing into another Mail, which you can't take.
