In the early stages you'll want to take every card you can in, before combining cards! You can take a maximum of ten cards into any single battle, so make sure you've fleshed out your deck fully, particularly when it comes to multiplayer fighting. Combine cards once you've gone over the ten card limit.

Note that if you aren't aware of the cards you're going to be facing off against, it's important to fight with a deck that maintains a good balance between all stats. Specialize in certain stats only when you have some intelligence about the enemy you're about to take on.