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), such as casting an incantation on an indestructible creature.
Keep in mind that the goal is to destroy the opponents Fort. 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 Fort.
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 Fort card. So you can regenerate your Fort.
Some other tips: Destroy creatures in the second lane only after you have destroyed the outpost via the first lane.