While that Megamorph ability sounds a bit costly, that's not an ability you're going to use until later in the game. It deals with a lot of threats that many aggressive decks would face, especially those may otherwise stonewall alpha strikes. Also, the Tarkir Warriors may become a viable archetype after Theros block rotates from Standard, especially with support like Arashin Foremost and Blood-Chin Fanatic also being in the set.
Anytime that you can get a two-drop that adds value later in the game, that's something that will probably see at least some Constructed play. Also, Hidden Dragonslayer isn't bad next to a card like Mastery of the Unseen. With Mastery, having this card being Manifested allows you to flip it face up for its original mana cost of 1W. (It would seem you could alternatively pay its Megamorph cost instead, as well.) If you flip it face-up with Mastery's effect, you still get the ability to destroy a creature with power 4 or more. So, there's some actual potential with this card even outside of the Warrior archetype, as it's not dependent at all in being played alongside other Warriors.