You really need to make sure you have your evolutionary lines complete. Here's how evolution works: You start with the Basic form. Next turn you place the Stage 1 form on it. The turn aafter that you place the Stage 2 form on the Stage 1 form. Brush up on your TCG rules, as you're missing Duskull, Ralts, Eevee, and Seedot. We'll take out the Gardevoir line, as it doesn't work quite as well with mixed type decks. We'll also take out Weavile (as it is likely to do little or no damage with its attacks), Umbreon (because it sucks), Musharna (since it isn't the best Pokemon and requires some self sleep in order to do anything special), Scarfty (there are just better and far more powerful Dark types), and the Shiftry line (definately not worth it for a Stage 2 and its ability can often be counter-productive. Also, multiple Stage 2 lines can be hard to pull off, and someone new to TCG should not be running multiple Stage 2 lines).
So first off, we need to make your evolutionary lines actually work. We'll add two Duskull, then add another Dusknoir to bulk up the line in case one of your Dusknoir falls in battle. To allow Espeon to be played, we'll add two Eevee.
Now for some cards that would work well with this deck. Bisharp hits incredibly hard with few Energy. ALthough your foe has to be damaged, Dusknoir can easily allow for that to happen. We'll add two Bisharp and three Pawnaird. Similar to Bisharp but doing less damage in exchange for attacking any foe is Mandibuzz. Again we'll add two Mandibuzz, along with three Vullaby. Finally, we'll add two Sigilyph to help you against EX cards.
With 23 Pokemon, I'm going to suggest running 17 Energy. While it seems Mewtwo would require the use of more Energy in your deck, Mewtwo also uses the Energy attached to the Defending Pokemon. And a total of nine energy attached to it and the defending Pokemon are usually all that you need to KO every Pokemon in the game. Four of these Energy will be Blend Energy which provide you with the Energy you need. We'll split the remaining Energy to be 7 Dark and 6 Psychic.
Now we have the capability of running 20 Trainers. I'd suggest taking out the Juniper. Some decks can handle the discarding, but I don't think this deck is one of those. We'll also take out Energy Search and Energy Retrieval, both of which are inferior to cards I'll be adding later. I suggest adding two more Cheren. I also suggest adding in one more Pokemon Communication. This will increase your drawing/searching power. To increase your drawing, while also meaning you won't discard cards, we'll add two N. We'll also add two Pokemon Catcher. Only two because with Mandibuzz and Dusknoir on the job, forcing out a Pokemon isn't needed to get the KO. However, I always say better safe than sorry. And they can get rid of a Pokemon that is a danger to your team. Now for the cards that are better than Energy Retrieval and Energy Search. While Super Rod doesn't bring Energy to your hand, this can be advantageous seeing as it helps in situations where you are facing a deck that is trying to deck you, such as my Active Stall deck. It also gets back Pokemon, in that unfortunate case where you lose basics. And finally, it gives you three energy instead of just two. We'll add four Super Rod to the deck. Energy Search only gives you one energy. Cilan gives you three. So we'll add three Cilan to the deck.
x1 Mewtwo EX
x1 Darkrai EX
x4 Pokemon Communication
x4 Super Rod
x2 Pokemon Catcher
x1 Gold Potion
x4 Blend Energy GFPD