Zoolock is the Hearthstone Deck I have been playing recently and it is a pretty solid deck, a diamond in the rough if you will. This deck has just as much pressure as it has always had, and with the slower OTK decks out there with less removal than is necessary for this fast paced deck with a ton of draw power, I think this deck is very good. It as a lot of things going for it and if you get Keleseth you win basically, and most people will think you are control and keep a slower hand which you can then take advantage of. This is a golden time for Zoolock I would recommend playing it.
Guide: Face is the place in most cases, but into another aggro deck you can trade in cause you can draw and just wait for the opponent to run out of fuel. It is bad into midrange Hunter especially because of draw power and Rexxar's endless amount of fuel but it still isn't even that bad if you get the Keleseth Tempo boost.
Mulligans: Keep basically all 1 drop minions, Keleseth, and Happy ghoul. Everything else you should dump to try to GET these cards so you can have a strong early game and maybe a free 3/3 to increase tempo at some point.
Battletags: Debatable#1963 (NA) / Brunselmant#1150 (EU)