Of course, for a 3 mana minion, the Emperor Cobra's attack strength and health are a little on the low end -- the cost of having such an impressive ability. You need to know how to play this card at the right time to make the most of it.
If you play the Emperor Cobra too early in the game, or when your opponent has a wide array of minions on the field, you aren't going to accomplish very much. At that point, your opponent might not mind trading one of their minions to eliminate your cobra. As it isn't very strong on its own, that situation is no different for your opponent than dealing with any other minion -- it's unlikely they'll grieve the loss of a Bloodfen Raptor.
It's late in the game where the Emperor Cobra shines, when your opponent has his/her strong, high mana minions on the field, and sacrificing one of them to eliminate your lowly 2 attack, 3 health minion is painful. If they have nothing on the field they're willing to sacrifice and play a lower mana minion to deal with it, it's too late by that point. That minion won't be able to attack on its first turn unless it has Charge. Any one of their minions is ripe for the picking.
Your opponent's other option is nullifying the Emperor Cobra through spell cards. In my opinion, that's the best case scenario. If a mage wastes a Polymorph, a rogue an Assassination, or a shaman a Hex on my Emperor Cobra, that's one less spell they have to deal with my beefier minions. You're forcing your opponent's hand. In Hearthstone, that's the best possible situation to be in -- don't allow your opponent to play whatever they want, whenever they want. Make them play what you want them to play, eroding their ability to be aggressive.
Paladins can apply a Divine Shield to the cobra, making it immune to its next source of incoming damage. Your cobra could then, in theory, 1-shot your opponent's Ragnaros and live to see another day. Cards such as the Sunfury Protector and Defender of Argus can also make your Cobra an unavoidable threat, giving it the Taunt effect.
Though the Emperor Cobra is a deadly, deadly threat, I wouldn't recommend building your deck around it. Include it, but don't expect it to carry you to victory on its own -- that isn't going to happen. Use the aforementioned combos when they're to your advantage, not the central theme of any deck.
Be warned: a rogue with Betrayal in their deck will ruin your day. Betrayal forces a minion to deal damage to the minions on either side of it on the field. Note that the Emperor Cobra's effect says it only needs to damage a minion to destroy it, not attack a minion. If a Betrayal is played against your cobra, it will murder its friends.
I was going to end the article right there, but I felt it worth mentioning that I misspelled cobra as cobro in that last sentence. I considered keeping it that way, but murdering your friends isn't a very bro-like thing to do, is it?