Turbo Darkrai and Darkrai / Giratina are 2 really powerful Dark decks in the meta right now. In the most recent regionals they made up 4 of the decks in the top 8. Considering you have 1 Darkrai in your deck already you may want to look at a deck like that. Turbo Darkrai especially requires very few Pokémon. Just Darkrai EX, Yveltal, and Hoopa EX. Here is a link with what the decks look like:
http://www.pokemon.com/us/play-pokemon/salt-lake-city-regionals-2017/tcg-masters/
Shaymin is almost certainly overkill. It's very powerful, but unless you plan on trying to win an upcoming Regional tournament, it isn't worth the rather absurd cost. Replacing those with a couple cheap draw supporters or extra Pokémon should do the trick just fine. With that in mind I will try to suggest changes for the deck as it is.
You really want your lines to be even with all your Pokémon. So I would change the Machamp line from 4-3-2 to 3-3-3. The main reason for this is you don't really need the 4th Machop if your 2 Machamp are already out and you have no Super Rods to get them back. If you are only running 2 Machamp, 1 of them being prized will hurt massively. Same thing goes for Pangoro and Umbreon. Also Lucario is Expanded. Idk the rules of your local place so it might mot matter, but considering you labeled the deck as Standard I will point it out just in case. In general I'm not much of a fan of 1 of Megas. It's usually very inconsistent and considering you aren't running any links I don't think there will be many times where giving up a turn will be worth it. Either make it more of the focus by upping the number to 2 and put in 2 links or remove it altogether.
Another thing would be to condense the amount of Pokémon. 22 Pokémon is a lot and having as many 1 ofs as you do will make the deck very inconsistent. Personally I would cut either the Machamp, Umbreon, or Pangoro lines. If you cut the Machamp line you would probably be best off going to Dark only. This is the direction I would personally go in since I find the fighting cards to be rather weak and Machamp to not be powerful enough to warrant 9 cards in your deck. Also adding a second Darkrai and Umbreon would help a lot with consistency.
Other cards to consider adding:
Yveltal XY / Steam Siege: Energy acceleration is really powerful. The Break evolution is also fairly powerful and worth considering.
Yveltal EX: Not as powerful of a card as Darkrai, but it is also much cheaper. Good for filling in the gaps if you want to run Turbo Dark but can't get the 4 Darkrai needed.
Hoopa EX: Gets you any 3 EXs that you want. Incredibly powerful if you are going in an EX heavy direction.
Oranguru / Octillery: Cheap versions of Shaymin. Each have their own flaws, but they can fill Shaymin's role decently.
Carbink: Blocks all attacks from EXs, which is a fairly powerful effect.
In the end the Pokémon I would run would look something like this:
2 Darkrai EX (if you can get 3 or 4 then just run Turbo Darkrai)
3 Yveltal EX
3-2 Yveltal Break Line (this is one situation where you can abandon the even line because Yveltal is powerful enough on its own that it doesn't matter if you don't have the Break, alternatively you could run no Breaks)
2 Umbreon EX (optional, you could run a 3rd Break instead)
1 Hoopa EX
The trainers is another place where you really want more consistency and better cards, especially draw supporters as Hau and Kukui are very weak at that task. Here is more or less what I would run for trainers.
4 Ultra Ball
2 Nest Ball
4 Trainer's Mail (helps with consistency a ton)
4 Max Elixir (amount depends on how many basics you run)
2 Fighting Fury Belt (again the amount depends on number of basics)
3 Switch or Escape Rope
2 Professor's Letter
2 Super Rod (good for reusing Darkrai)
4 VS Seeker (a rather expensive card, but it is incredibly powerful. If you can't get these then just increase the number of each supporter you are running)
3 Sycamore
3 N
2 Lysandre
1 Hex Maniac
1 Pokémon Ranger
3 Parallel City, Reverse Valley, Chaos Tower, or Silent Lab
12 Energy should be more than enough. You will find having good draw supporters like Sycamore and N will make finding Energy much easier.
Hope this helps.
