Energy can take a long time to build up to anything, but with a wall of structures up it can be hard to do anything against.
Spells like Iron Whip, Burn and Spark will help you target enemy units wherever they are. There are some low cost automatons that can be a real headache as well.
The game does take a lot of thought. Even once you have a deck you're happy with, knowing when and how to sac cards can turn a game completely. Plus there's also the joy of having a really crappy draw to begin with that can completely end you before you've even started...
Play some of the trials. Even if you don't win, you can pick up some good ideas from the ai's tactics. There's one trial called "destruction from above" or something like that - it's the last Easy one. The ai plays an energy deck and I cannot for the life of me beat it.
It does already have 3 catapults of goo in play before you even start, so that's quite a wall to overcome before you even start!
Willow Ravenwolf, Fyanna; GW2 (Yak's Bend) [Willow Ravenstar; Guild Wars and Star Trek Online]