I have to say that it is probably not as powerful as you might say. For a few reasons.
You have only 1 supporter as far as i can tell. Idk what some of those cards are. You need draw power to make your deck a true threat. You have way too much energy. 18 is way to high. Probably 10 to 14 is average. you have too many pokemon and too many different pokemon lines. You main strategy as you said is to power up shaymin ex by just throwing pokemon under the bus. The only problem with that strategy is that all they need is one good attack on shaymin and it will go down for 2 prizes. The only time you really use shaymin is after you lost maybe 4 or 5 prizes already and you are going for your last prize as well.
Either way your deck is just way too inconsistent. With pokemon, consistency is key. If you play against a deck that is set to get its strategy going by turn 2, you will most likely lose. Lets imagine how a quick game with my favorite deck would go.
Turn 1 i will usually have either tropius or weedle in the active, be able to place down an grass energy, search or play another weedle, use an N, Juniper for 6 or 7 new cards, or Skyla for a key card i need to get set up giving me anywhere from 8-15 cards to play around with. Turn 2 i will most likely have a beedrill out with a rare candy doing 60 damage for 1 energy along with 30 poison damage with confusion. Turn 3 and 4 i will probably get 1 or 2 more Beedrills, claim 2 prizes by that time if you haven't played your EXs, in which case i would have taken more. I would also have played another supporter or 2 by that time giving me access to more cards anywhere from 2-14 more cards in those 2 turns depending on what supporters i drew into. Cards drawn by turn 4 is probably around 30 on average.
Turn 1 you will only have access to the your 7 cards you start with and the 1 you draw for the turn. You probably start with 2 or 3 pokemon and 2-3 energy based on the counts. With 1 or 2 trainer cards. Its hard to say which ones you would have since you only have like 1 of each. You play your pokemon and the 1 energy for that turn, maybe attack idk. Turn 2 you still have the same pokemon, maybe draw into a different basic or 1st step, possibly evolve and play another energy. However since i attacked you have no choice but to use your energy and retreat, or sacrifice your current pokemon to start powering up a new one. Turn 3 and 4 you have only drawn 11 cards total and are behind on prizes because you keep having to sacrifice pokemon to power up the one on your bench.
Access to more cards allows your to get your strategy going faster. More consistency in cards also allows for the strategy to be faster and more reliable.
You may think, i only have 10 energy cards in my deck, how do i ever get them when i need them? I use a supporter. Professor Juniper allows you to draw 7 new cards to your hand. If you take out the first 8 cards you draw and the 6 prizes that leaves you with 46 cards left in the deck. So if i draw 7 new cards there is a 6th of my remaining deck. With a 6th of my total deck being energy that gives me a pretty good chance to draw at least 1 of them.
