Mewtwo ex and azumarill

Here's a few tips from me for your league, if you don't have the cards then you can try to get them later.
1. You really don't need three Mewtwos. I know it sucks not to play them when you traded three, but they are more of a liability because they are easy to knock out, and it takes at least two perfect rounds *after* you have all your Xatus in play until they start being able to knock out weaker ex-Pokemon.
2. On that note, reaaally try to get another Natu and Xatu. I tested this deck a lot, and it often looks like you don't need 4 in play since you don't need to be super fast with adding energy due to my addition to the deck, Azumarill. However, there are some matchups/hands where you will just not be able to get an Azumarill into your game quickly enough, and/or where your Azumarill is useless, because it doesnt do enough damage and/or because your opponent will just use Counter Catcher or Bosses to KO your Mewtwo. It is very important as a draw engine, and it is very important as a way to very very quickly set up Mewtwo as well. I promise you won't regret it.
3. In the beginning, I also used to play 4 Arvens and 2 TM Evos. It's a nice idea, and I'm so used to it because it works so well in Gardy for example. But surprisingly, this doesn't work super well. Why? In Gardy, you now have two Kirlias, get to draw 4 more cards, might even evolve more Ralts' and also get to evolve into Gardy easily, and either attack with Gardy directly or literally with any other basic pokemon in your deck.
In this deck, you need to decide between evolving into two Xatus, or one Xatu and one Azumarill. If you evolve into two Xatus, you need to be lucky enough to have two energies in hand. Then you need to find both Mewtwo, and Azumarill, or a card that can find them. If you find Arven, you can only do one of the two, and also that was your supporter for the turn. You also don't have a stage 2 pokemon, so wasting that space of needing two cards (Arven and TM) to evolve two Pokémon is what makes this unnecessary. Instead of doing all that, it's much smarter to build a deck without Evos, and replace those evos with more consistent ways to get all the cards you need by turn two. Jacq for example also gets your two evolution pokemon, while only being one card. Ultra Balls are your friend. Ciphermaniacs Codebreaking is good, too, especially if you already have the setup to draw two right after. Also, you don't have EXP Share *or* the Maximum Belt, so after you finished setup, Arven is a one item draw supporter, which would make it pretty much the worst supporter in pokemon tcg history.
3. The Area Zero Underdepths are a good idea to spread the energies in theory, but in practice, you can literally put 3 Energies on each Xatu and your opponent will still rather snipe your Mewtwo or Azumarill than sniping any of those. Plus, if your opponent switches out the stadium, that energy is gone now either way. EXP Share is much better at making sure any energies attached aren't discarded. With Tulip, you can get any rest back and immediately redistribute with your 2-4 Xatus. Try to get EXP Share into the deck instead if possible for that reason

4. Four Ionos is a must. Not only is it a great disruptor, it's also your only!! draw engine besides Xatu. If you have a slow start and can't find Xatu *and* energy, you neeeed Iono or Professor to make sure you can catch up. Iono and Professor also have great Synergy with Ciphermaniacs Codebreaking. Imagine you are second to play and you only have a Ciphermaniacs Codebreaking in hand, and no way to both evolve into a Xatu and have an Energy in hand (I promise you, it will happen. Some hands are ROUGH). Now you can play Ciphermaniac, put Iono on top and a Xatu or Energy underneath. Then, next turn, you not only get to play Iono to both disrupt your opponent *and* get rid of your hand, but *also* get the other card that was underneath Iono since Iono shuffles your card under your deck, not into your deck. So 4 Ciphers and as many Ionos and maybe 1-2 Professors will save your behind.
5. You have 12 energies. That is one fifth of your deck. You don't need a third Earthen Vessel. You can literally get 6 of your 12 Energies with those Earthen Vessels, I promise you that you will not have a single game where a third Earthen Vessel will be of benefit over a second Ultra Ball.
6. If you implemented all this and still have space, get another Pokemon. In the beginning I had Munkidori (Twilight Masquerade) with the idea to heal my Azumarill and at the same time spread some more damage. It's not super consistent with the Darkness Energy though, and the Moonlit Hill (Paldean Fates) stadium does the trick too (can really recommend it!). Cresselia (Shrouded Fable) is good too because you can quickly power it up and make some damage if you need to look for a new Marill to put into play and evolve. I tried Deoxys (Paradox Rift) too because it puts away the energy, but EXP Share makes this just a worse Cresselia. Now I play Flutter Mane (Temporal Forces) because 90 damage is also a lot, and its ability disrupts/stalls. Any other stall cards like Klefki (Scarlet & Violet) or Mawile (Lost Origin) or Mimikyu (Paldean Fates) are good, too.

maksi Hi, totally agreed with your suggestions, I used cresselia and it works fine, unfortunately I couldn't find the exp. share yet, but testing it on tcg live it works fluently with that since mewtwo can attack second after azumarill

maksi in my case maximum belt being replaced by 1 Black Belt's Training (Prismatic Evolutions) works fine since most of the time azumarill just hit 1 time, it is not the same amount of damage but it works must of the cases when they evolve into dragapult or they are using Roaring Moon ex (Paradox Rift)

in conclusion, since mewtwo is one of my favorites pokemon, I will push this trough the rotation to see its potential, thank you so much for your suggestions I really loved the deck you made! maksi 

Maybe playing a few more Psychic Energies? Mewtwo can deal more damage with more Energies. Hope i helped!