There are a few big errors in here that contradict the theme of an EX-only deck. The main fault is playing Revive in a deck with four copies of every EX you play. Since you will give up two prizes per EX, your opponent will win before you have the opportunity to play your fourth copy let alone the fifth and sixth copy of your Pokemon. Rescue Energy falls under the same umbrella as Revive does, as both will end up giving you Pokemon you won't be using against your opponent. Psychic Energy also is not going to be effective in this deck, as Mewtwo EX is used for its first attack, X-Ball, which can do massive damage for few colorless energy. Prism Energy, which allows decks to play a greater variety of Pokemon types, will also be insignificant in this deck because the deck only has one energy-specific attacker, Groudon EX.
A couple generally weak cards that I see here would be the Interviewer's Questions, Cheren, and Sage's Training. Interviewer's Questions gives you only a chance to receive Energy cards. You may miss out on an energy that you really need--more than likely, it will be a Double Colorless Energy--and find yourself holding a lackluster set of cards and energy. A better version of Interviewer's Questions is Energy Exchanger, which is a reliable way to get any energy you need per situation as it lets you search your deck for Special Energy cards.
Cheren is not very powerful because it does not give you a great variety of new cards to use per turn. There are more powerful cards that are legal for play at the moment, such as Professor Oak's New Theory, N, and Professor Juniper. Those three cards make up the supporter base of most competitive-quality decks, and they should be what you ought to use in any deck you build. With the support of PokeGear 3.0 and Random Receiver, the three supporter cards I mentioned are better options to play in any deck.
Sage's Training is one of the few supporters that fits perfectly in some decks. Stage Two decks like it because it allows the deck to add two of five cards into their hand without "refreshing" it like N and Juniper do. That will hopefully give the player's hand cards like a Rare Candy or the Stage Two to evolve his or her Pokemon into. Decks that work with discarded energy in the early stages of the game also like this card, as it is a reliable way to discard those energy cards without needing one previously in the player's starting hand. While some decks hurt from the loss of important cards, Sage's Training is played with much success in Magnezone Prime decks (for the double purpose of fueling Eelektrik, which makes the card fit in perfectly) and Landorus decks, as the benefits of Sage's Training come from both the discard and the drawing effect.
The choice of Pokemon in here is rather arbitrary. Mewtwo EX should not be played in such a large amount because of the card's own popularity. It's weakness to itself makes it a detriment of a lead Pokemon if it's the only Pokemon in your starting hand, forcing you to play it earlier than it is best used. Regigigas EX is also very vulnerable to fighting type decks, which have risen in popularity due to the most successful decks being lightining- (Eelektrik) and dark- (Darkrai with Dark Patch)based. It's a good card, but I recommend that you use less in here. Groudon EX is a good card overall, and its weakness is not a commonly played type, so playing four isn't a bad idea. The Rocky Helmet also has outstanding synergy with Groudon's Giant Claw attack, and it also softens up the opponent's field when combined with Tromp. It seems the focus of this deck best belongs to Groudon EX.
Using support Pokemon for a fighting deck like Landorus would help the deck run smoothly with the energy added to the field, the second turn attack, and the helpful damage spread to activate Groudon's Giant Claw's added effect. Terrakion is also an easily energized attacker that hits the "magic number" 180 to knock out Darkrai EX for only two energy. It is easily fit in this deck, and it provides a great threat to Pokemon that are weak to it. Shaymin from Unleashed and EXP Share are also strong cards as they keep energy in play. This is especially important for this deck, as you have no effective single energy attack and no powerful double energy attacks (besides Mewtwo EX). Dual Ball is a good card for getting whatever Pokemon Heavy Ball can't give you. Junk Arm also helps with discarding and increased consistency. Switch lets you ignore retreat costs in this deck when your Pokemon need to find safety on the bench, and it combines well with Shaymin by letting you move the energy from your incapacitated Pokemon to fresh Pokemon. Although you may have to look up most of the cards mentioned, they come together to form a very strong deck:
Pokemon: 12
3 Landorus (Noble Victories) (A great starting Pokemon with a potential second-turn 80 damage with 10 damage spread to the field)
3 Groudon EX (Dark Explorers) (A good starting Pokemon due to the strength of a second turn Tromp and combines well with Landorus. It is also a very big threat to the current set of popular decks with its type and damage potential)
2 Terrakion (Noble Victories) (Easy to attack with and becomes an immediate threat to Darkrai EX and Eelektrik-based decks)
2 Mewtwo EX (Next Destinies) (Counters itself and may win entire games on its own versus decks with no Mewtwo of their own)
2 Shaymin (Unleashed) (Keeps energy in play and lets you quickly power up fresh Pokemon)
(Regigigas EX was excluded from the deck because it provided nothing important to this deck's function and was not vital to this deck in any match-ups [Match-up a term used to suggest a deck in a game versus another deck usually to compare one deck's performance versus the other; ex. the Darkrai match-up or the "mirror" match-up, a deck versus a varient of itself])
Trainers: 34
3 Professor Juniper (Very good in the early game when you are able to play every card from your hand and discard a fighting energy while Landorus is active. It takes skill to play it in most stages of the game, however)
4 Professor Oak's New Theory (As risk-free of a supporter as you can get)
2 N (Your fifth and sixth "PONT" and allows you to seal games late in the game if you have a strong field)
2 Sage's Training (The best card to have early in the game if you plan to attack with Landorus. It's not bad to bump into with Random Receiver, either, and any unused copies can be discarded with Professor Juniper or Junk Arm so you can make Random Receiver more effective)
2 Random Receiver (Turns Junk Arm into a consistency-boosting card)
(The above cards make sure the deck runs consistently)
3 Dual Ball
3 Heavy Ball
(Dual Ball and Heavy Ball give you the Pokemon you need when you need them. When combined with Junk Arm, the variety allows you to retrieve whichever card you need when you need it. Heavy Ball is played because it gets you Groudon EX and Terrakion more consistently than Dual Ball does, which risks failing by flipping two tails)
4 EXP Share (Prevents you from running out of energy and combines well with Mewtwo EX by keeping it safe from opposing X-Balls until you plan to attack with it)
3 Pokemon Catcher (Overall a fantastic card by letting you take prizes during more turns of the game)
2 PlusPower (Negates Eviolite, a damage-reducing card, on Darkrai EX and Mewtwo EX when hit by your Terrakion and Mewtwo EX respectively)
1 Potion (Avoids Darkrai's 2-hit KO on Groudon EX without added damage from Special Dark, Dark Claw, or PlusPower and greatly hinders lightning decks' attempts at KO's)
4 Junk Arm (Extraordinarily versatile, it lets you customize your hand for certain situations)
2 Switch (Lets you bypass retreat cost and keeps energy in play)
1 Super Rod (Lets you recover much-needed energy and Pokemon, but it very easy to remove from this deck)
Energy: 12
10 Fighting Energy (The most important card in this deck. The number allows you to retrieve them during most of your turns in conjunction with the supporter cards in your deck)
2 Double Colorless Energy (Allows you to power up Mewtwo EX's X-Ball in a single turn and is sometimes effective in retreating)
I hope this semi-article teaches you something about the competitive side of the Pokemon TCG. If you have any questions regarding any aspect of the TCG, I'd be more than happy to answer them.
