I always assumed that the auto battle system would choose the best combo available, but I never checked it, so I might have to choose my card deck more carefully.

I will also look at the skills more. I was mostly looking at the attack & health.

Is it better to fuse every possible card, to make them stronger? Or does it make sense to keep doubles, like two Epic Riders instead of one fused?
I often see cards with level 12 and higher, so I suppose most people fuse their cards?