Hitting enemies, changing the battle and its outcome, is fun. Generally, the more dramatic the effect, the more fun it is.

By contrast, missing or otherwise failing to impact a battle (discovering an immunity or high resistance the hard way) is annoying and not fun. Perversely, this can make successes feel more significant and hard-won by comparison.

Too much missing, though, becomes a problem. Players will start finding other things to do with their time.

High defenses on powerful enemies certainly pushes things into the not fun direction. Changing the badly wounded powerful enemy to be a nearer-level Solo Brute is the best option.