Hearthpwn

There are a lot of different decks to defeat Coren Direnbrew, but the basic tactic will most of the time be the same. Mulligan for removal spells and cheap minions and let the Hero Power of Coren put the big minion on the board for you. His minions won't be able to attack when they are summoned, so take advantage of the situation. Trade efficiently and push for damage when you can. 

I defeated Coren on my first try with this deck. As you can see in the video I got a little bit lucky with my Ragnaros the Firelord, but this first boss of Blackrock Mountain shouldn't be that hard to defeat.