Sorry! I completely forgot to rant about the deck you might be playing for worlds. So let's get down to analyzing it ^^
First of all, I'm going to assume you're playing the deck the way I would for any deck without energy acceleration for Terrakion and Mewtwo, so running 4 Energy Switch, 2 Shaymin, perhaps a few super scoop up, a clunky 2-3 Mewtwo line and the card central to techs, a single energy search.
Darkrai/Terrakion/Mewtwo is an incredibly strong play for Worlds. With the amount of ZekEels and Darkrai decks that'll be played at Worlds, you're playing a mainstream anti-meta deck, in a sense. You're immediately at a disadvantage with regard to Mewtwo wars because of the inferiority of your energy acceleration, which requires you have a Shaymin in hand while also having the bench space required to drop it. An alternative for Mewtwo wars is attacking first with Darkrai. As long as you attack Mewtwo first and can consistently do that for the rest of the game you should be in the prize lead. If they get greedy with their Mewtwo you can respond with Mewtwo Energy Switch + Attach, not having to waste a Shaymin.
Terrakion is a very bulky basic that can hit very hard in clutch or unexpected situations, and its tank potential is represented by its retreat cost, ahah. Darkrai fixes that without having to us Switch, so Terrakion is a stronger tech in Darkrai than any other deck. Any simple energy switch + energy search combo can set it up quickly, and you can even go for the immediate Land Crush if you used Fighting energy to cover the attack cost of a Darkrai with it. You can abuse the use of Raikou and Zekrom EX in ZekEels to take two prizes and secure the game for Mewtwo or Darkrai. The great thing about using two techs in the same deck is killing two birds with one stone, you can set up both Mewtwo and Terrakion with a single Shaymin and decide which one to use for the current turn and rest of the game.
Definitely go for it, stand-alone anti-meta decks like Terrakion that take 2-3 turns to set up won't see a lot of play at Worlds but Terrakion is still a popular tech, and I'm guessing you have your heart set on playing a Darkrai deck.
I wish you the best of luck at Worlds.