I have the demo, it's pretty good, but I'm enjoying the card game too.
There is strategy to it demi, though to proceed in the colosseum you totally need a few max level cards, but eventually the strategies revolve around ability choice (you can only have 3, and they get new ones at 15 30 40 50) and order of deployment, element selection. You kinda have to cap a few cards to really compete though.