You shouldn't need a deck that holds more than 30 cards.
First, go back and do all the side quests you probably skipped in Krokotopia and Wizard City, so that you gain a few levels. Don't skip this part. By the time you've done all of the quests, you should be close to level 28. At level 28, you learn Tempest, which will more than even the playing field for you.
Second, be sure you've learned all of the spells you have coming. If you don't have Kraken and Water Elemental, visit Dworgyn in Nightside. If you don't have the following spells, you can find them:
Storm Shield: Sabrina Greenstar the Fairegrounds in Wizard City Commons.
Soothe: Mortis the Death Tree (Nightside)
Dissipate: Mildred Farseer, Colossus Boulevard, in the house down the street from Mindy Pixiecrown.
Sap Power: Croaky in the Digmoore Station basement in Marleybone.
One thing that will help is to use your Water Elemental until you get Tempest. The minion will use Taunt to direct enemy attacks towards it, so you can build your attacks. However, that isn't a substitute for going back and doing all of the side quests. You'll want to get to level 28 as fast as you can, and the quests in Krokotopia and Wizard City are easier to finish.