Heroic General Drakkisath is probably worst realized heroic challenge in this expansion till now: totally based on luck without any required skill...
I haven't tried heroic yet but I found Rend was the most difficult challenge yet for the normal difficulty. Took me a lot of different decks and tries before I beat him.
For heroic Rend I found that a mill Rogue deck works well. Replace a lot of your typical cycle cards for healing like health bot and earthen ring farseeer. Took me a few tries but once you gain control of the board you're vanish and sap's can really maintain it well before he gets to fatigue.
Finally beat Heroic Rend with a Hunter deck. Explosive Trap can be devestating if you get it when you need it and, with so many minions on his side of the board, plus and can really help. It still took a lot of tries but I managed to buff my Hyena up to 24 damage in the end, for the kill!