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i realy like your idea ill try it thanksHello there! I get that you're trying to boost Marshadow GX's damage with the fighting support, and while it's a cool idea I don't see it working the way you'd like it too. I'm not trying to discourage you or convince you to play something that you don't want to though. This game is meant to be fun and by all means play whatever makes you happy! That said, I would at least like to offer some advice on what you've got here
Firstly, let's address the main issue. Marshadow GX only has 150 HP but still gives up two prizes upon being knocked out, making him not so optimal as a main attacker. He's a perfect tech in for decks as a late game attacker, especially against decks like Darkrai EX/GX and ones that use Tauros GX or Drampa GX. However, if you are set on using Marshadow GX as a main attacker then I'd recommend using Fighting Fury Belts over Choice Band. They were released in Break Point making them legal post rotation, still give you a bit of a damage boost (albeit only 10 damage), but more importantly boost your HP by 40 giving Marshadow GX a better looking 190 HP.
Next we have the Machamp line and Regirock EX. Sure they can boost Marshadow GX's damage output, but at what cost? Both have less than ideal retreat costs with little to no way of reliably retreating them, cannot attack due to only running dark energy, can have their abilities shut down with Garb or Hex Maniac, and only help out Marshadow GX making them dead-weight to your Darkrai. If you are adamant on having a damage boost for Marshadow GX then I'd suggest stick to Regirock EX. Regirock EX may give you less of a damage boost, but being a basic means you don't have to go through all of the evolution line needed to obtain it.
Regardless you'd need a reliable way to retreat him if he gets pulled into the active. Consider running two floats for free retreat or running three/four Altar of the Moone to make anything with dark energy have two less retreat (in Regirock's case making his retreat cost only one energy).
The next issue I can see is with Marshadow GX's ability. Since it can only be used on a discarded basic, and as this deck list stands there are minimal ways to discard your Pokemon, you'd need to increase your chances of getting a Darkrai EX into your discard pile. Consider bumping your ultra ball and sycamore counts to four each, or even running a few of the new supporter, Sophocles, from BUS (he let's you discard two cards from your hand to draw four).
Your current list only has two expanded cards in it. If you drop the seekers and the trainers mail you'll have yourself a standard list.
Once again, I don't want to discourage you from playing a deck that is fun for you, but I would like to suggest a more consistent variant of your deck list.
Here we go:
Pokemon (13)
Marshadow GX x3
Darkrai EX x4
Tapu Lele GX x2
Regirock EX x2
Drampa GX x2
Items (16)
Ultra Ball x4
Fighting Fury Belt x4
Field Blower x2
Rescue Stretcher x2
Max Elixir x4
Supporters (16)
Sycamore x4
N x3
Guzma x3
Pokemon Fan Club x2
Acerola x2 (BUS)
Sophocles x2 (BUS)
Stadiums (3)
Altar of the Moone x3
Energy (12)
8 Dark Energy
4 Rainbow Energy (still counts as dark energy, helps out to use Drampa's Berserk, and can count as a fighting energy if necessary!)
Marshadow GX can still be the star of your deck, still have some boosted attacks, be more consistent, and even played in all formats!
Good luck with whatever direction you decide to go and have fun playing!
-L0L