If we are talking about "Rainbow" Marshadow Box (opposed to the more "Fighting" oriented version), it is a good deck but it has some flaws:
- In standard since you cannot use Prism Energy, running the deck is clunky. If you run it with Strong energy, you gain damage but the deck gets clunky since you cannot attach to every attacker in your deck. Because it works with a discarding engine, this makes it once again more clunky (you cannot always discard the Pokémon you need and can also end up with a Marshadow active without good attacks to use). To sum it up, the deck will always be more or less clunky to handle.
- The deck is weak to Energy denial and as few ways of accelerating energy (Shining Mew SM4 or Carbink Turbo XY). Magearna-Ex helps against Drampa-Gx matchups since it neglects Righteous edge's devastating effect. In standard only having Rainbow Energy reduces the odds for you to be protected through Mystic Heart. This makes Drampa-Gx a harder matchup. Mill decks are a big problem for Marshadow-Gx for those reasons. Playing against a Sableye DE or Sylveon-Gx deck is a Nightmare.
In my opinion, given that it is clunky and is weak to energy denial, this deck is good but not good enough in the current meta. It is definitely fun to play since it offers so many options and once it gets going it is very hard to stop. It is not a straightforward deck and it is hard to master. You are given plenty of choices and mistakes can easily be made. I would say that it is a high risk high reward deck.
I cannot give any answers to your problems regarding standard format. The deck is what he is, very fun to play but frustrating because of those consistency problems. On the other hand, if you like the playstyle and feeling of the Marshadow Box and you play expanded format, you should try the Ninja Box deck list i came up with. Some will say i am a scrub and a stranger with no credibility and you are free to believe them and not try it. But the deck i made comes with the same feeling that the Marshadow Box gives and answers some of its flaws.
First off, Electrike Primal Clash (or Regirock XY Promo) is a very good way to avoid the problems related to energy denial and make sure that you do not fall behind in this crucial aspect of the game. Playing Ninja Boy is crucial if you do so because obviously Electrike will not be the attacker, he is only there for the energy stacking and preservation. Having this Pokémon in your deck actually makes it possible for you to beat Sableye DE or Sylveon-Gx as long as you do not bench High retreat cost pokémon. You could include it in a Marshadow-Box but it is usually sub-optimal and less reliable since you do not run much Ninja Boy copies.
Secondly, while Marshadow Box runs on a discarding engine, Ninja Box runs on a "in your deck" mechanic. You need those pokémons in your deck (except for pokémon supporters ofc) in order to use Ninja Boy effectively. This "in your deck" mechanic is less clunky (it still has it's flaws but this is partly why i play Karen and 2 Rescue Stretcher) than the discard mechanic and get's the deck online faster. Marshadow Box will usually become a more powerful deck than Ninja Box later on in the game in the sense that you do not need to Ninja Boy each time you want to play a specific attack, you can use all attacks once you discarded everything.
Finally, Ninja Box comes with some added sugar on top; you will hit for weakness more often than Marshadow Box since you will be able to change your active pokémon type.
Some pokémons are better suited for the Marshadow-Box (Jirachi XY Promo for exemple), some are better for the Ninja-Box. Latias-Ex Plasma Freeze is a pokémon that can only be used effectively in the Ninja Box deck. This pokémon makes it easy to beat Trevenant XY decks (unless they are playing Silent Lab stadium) while Trevenant is much more difficult to beat with Marshadow Box. Latias-Ex is also a counter to Marshadow-Gx since he can attack trough flash Ray while being immune to Marshadow-Gx attacks). While this is true with Marshadow-Gx, this will also be the case when Xurkitree-Gx is released.
By playing Ninja Box, you play "outside the box". The Ninja Box is unexpected and has clearly not been taking into account by designers since it runs such an odd drawing engine. Be sure that Marshadow Box on the other hand as been released well knowing that this engine would be played and that there would be counter to it being released shortly after. Ninja Boy is just the most versatile supporter ever printed (used for drawing/retreating/bench manipulation/Toolbox attacker and probably some other things i forget) and this allows the deck to be fluid and functional in most aspects of the game.
