Hearthstone: Heroes of Warcraft/Tutorial

You begin the game as Jaina Proudmoore, a mage. The initial six battles of the game are essentially a tutorial. Each match is designed to introduce new concepts into the game.

Illidan Stormrage
At the start of this fight, it says the fight will be unfair, so blame the developers. Illidan is significantly stronger than the heroes you faced previously, and the match will start you out with a disadvantage (you don't have any cards to play on the first round!). You will take a beating before you come back, but this match is beatable.