One thing I discovered in my own first run-in with a Beastmaster (I received my sets Wednesday and played my first games Thursday) is that they sometimes become so enamored with their swarms that they play their mage overly aggressively.  If your opponent fails to set up a Guard around his Beastmaster and sends his swarm out on full offense, my suggestion is to focus (assuming you've already pulled out a few creatures yourself) on cursing the Beastmaster and sending your creatures in (and possibly your Warlock) in for the kill.  Especially at apprentice levels of life, it doesn't take much damage to put down a Beastmaster, especially one stuck with the relatively weak non-creature spells found in their apprentice spellbook.