I think it'd be kind of cool if leveling up those Enemy Cards had more of a point than just boosting exp gains a little. If getting the entire set leveled up unlocked a new zone with beefed up versions of the killed enemies, I think that'd help make the cards feel like a fun side form of progression. Maybe even have a teir of unlocks, like level 5 gets you one set of enemies, lvl 10 gets you an even stronger set of enemies etc.