2294207:
He took 2 or 3 raid decks and maxed out the card limits for the best cards.
Just to note. When you do customise a raid deck, you may only include in cards which are originally from that raid. As such, you can't put in Onyxia or Molten Core raid cards into the raid deck. Also from the group size, there is a fair chance that what does kill you is the card, Toss About'. I suggest that your mage is dedicated in interrupting such cards and that your warrior/hunter would also run interrupt cards to support the mage when he is down.
Aside from that, Myriam + Inspire: ally works incredibly well... provided you ignore the part that HFI is not included in yet. Even then, Shem or Naseth can actually put a very quick burn on Magtheridon considering your play group size. Remember that you may ready the ally which means that you could actually choose not to when your allies are holding onto the cubes.
Some people learn, some people don't. For me, I can never correct myself to say Julia Graves as opposed to Julia Roberts