Decks in Magic Duels must contain between 60 and 100 cards, but you should always opt for the minimum as it increases consistency of the deck, as adding too many cards hurts your odds of drawing good cards.

Typical deck has around 24 creatures, 12 enchantments, sorcery or instant cards, and 24 lands.

Don't stack up too many high-cost creatures with a converted mana cost of five or higher, two or three should do the trick.

It's wise to unlock the Magic Origins set first, as the cards in it are very versatile and will work with various decks.

You should aim to save your removal spells for the later stages of the game to remove bigger threats.

As for the majority of freemium games, you should play the campaign as it will give you enough gold to buy packs, and enough experience to battle valiantly with real opponents.