A general tip in these deckbuilder games is to try to focus on a mechanic. Most classes will give you an idea of what they focus on with their starter cards. Sometimes it requires waiting and seeing what cards show up.
For example, I had one build which revolved around gaining strength, increasing damage temporarily and multi-attack cards. Getting +17 to all damage is much more devastating when applied to four hits (2 base damage x 4 + 17 bonus damage on each hit x 4).
Don’t skimp on defense and mobility unless you can kill enemies before they can attack you (like the ranged hunter class).
You should try to add cards with good synergy or better quality to your deck and remove cards which don’t work well or are lower quality. The less cards you have in your deck, the more often you can draw the good cards you want.
This is also a fairly hard deckbuilder by deckbuilder standards, since it’s also a tactical RPG with pretty good AI on top of it. Figuring out positioning to prevent getting teamed up on by multiple enemies at once and getting movement cards are essential in addition to building a good combat deck.