My interpretation of what defeating someone at specific difficulty means

[Takos]

Then what would fighting a difficult opponent but not getting injured be categorized as?

What do you mean by "difficult"? If you don't get injured, but the fight takes a long time (I guess that's what you may classify as difficult), it's mid difficulty according to my interpretation.
But if by "difficult" you mean that you got on the verge of death/losing during the fight, it's high difficulty.

Did you really put Itachi vs Kakashi fight was mid difficulty?

Yes. Itachi felt he wouldn't be able to deal with Kakashi without using a health-deteriorating technique, which classifies as winning but not without side-effects.

An example of no difficulty is in Naruto X man's signature.

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