But even if we do face most enemies only once, a partial solution is to allow additional nature/arcana checks after killing each enemy. As long as there are multiple enemies of that type in the fight (pretty likely, unless it's a boss-type enemy) then as the fight progresses you'll learn more about how to better kill each enemy.
Ok, let's say we do that. And what action do I use to make that check?
* Action - seems excessive, I could just attack another creature.
* Bonus action - can be the case, but classes that have little to do with bonus actions will benefit from this more
* Reaction - may work, but reactions can be incredibly powerful for certain classes
* Free Action - seems like the most fair one to me, but at that point why roll at all? Plus, if you roll an check and roll badly, you may never know how hard it is to kill a creature.
For me it creates more problems than it is worth, that's all.