(I know nothing about Magic the card game)
I've been unlocking all the cards for the white and black deck. The hints say you should be aggressive when playing against white, and white is defensive. The white deck in the game seems to be rush offensive deck. With Holy day you can commit all your flyers to nonstop offense, and then be able to stop any large attack on yourself. If you are building up cards it's pretty good against the first green and blue deck.
The black deck is a fairly defensive deck. You want to put out the rack, underworld dreams, megrim, scoot imp, and demon's horn. I usually sacrifice any of my creatures if I can kill something to keep the game fairly creature free and just let the special cards killl your opponent. It's ridiculously good against the first blue deck, so it's very easy to get all 17 cards.
Seems like a lot of people play the elf deck online. That's a pretty good deck, but I can usually kill it with the white deck. I beat this one guy (elf deck) 31 to 0, I pulled ridiculous cards though.