Clash of Decks can be a bit puzzle-y. Take some time to calculate what will happen when you play your cards.
Make sure you know the cards and the symbols. Many games are lost by forgetting about the effect of certain powers (symbols).
Keep in mind that the goal is to destroy the opponents fortress. To do so you must first destroy the outpost. It is easy to become distracted and start focussing too much on destroying the opponents creatures. It can be a very good strategy to ignore a lane in which the opponent has creatures that do little damage to your fortress.
Furthermore note that if you play cards that are to the left of your fortress card, when they are killed or used they come back to the right of your fortress card. So you can regenerate your fortress.
Some other tips: Destroy creatures in the second lane only after you have destroyed the outpost via the first lane.
look at your opponent cards before starting your turn. I use an anihlation or something on an indestructible way too many times