The basic rule is that more cards stacked together, the more powerful the attacks you can deal out will be. You will be limited to a maximum stack of three at first, and further limited to how many combinations you can, um, combine, in a single turn.
But you’ll quickly be able to experiment with even more attacks and abilities. It might be better for you to use a single card in some situations, but in general, stick to the bigger combinations, if possible.
You can choose which skills are best to use given your situation – you’ll often find wider-hitting AoE type attacks will be more useful.
If you’re going to be battling using Auto mode, the Skill menu also allows you turn off certain skills which are less useful, forcing your characters to either be more conservative or more hostile.
You should honestly hold on to your relics if possible, and avoid using them. They have powerful effects, including the ability to wipe enemies off the map instantly, so they’re best saved for when you’re in trouble.
Save up those Relics for the fights where you have already lost once, so you know that using them won’t be a waste.
You will need to jump into the Hero menu and level up heroes manually with gold and other materials.
This isn’t difficult or anything, but you should be conservative when spending level up materials, especially when it comes to upgrades. Upgrading is necessary to continue levelling your character past certain thresholds, and if you don’t have the materials for the upgrade, you’ll be lamenting it.