@Tulin1 For me a hard guest is someone who arrives, has not read the description/amenities, has an overwhelming attitude of entitlement, and expectations that are completely unrealistic, and they decide it is all your fault and lash out, such as writing a dishonest nasty review with a low star rating.
You do not say what they are complaining about and whether it is valid but out of your control, so it's rather difficult to give any advice. In general, I would suggest, do not take it personally, and if you have done what you can do within reason (if you can do anything) and they persist (and their stay is longer than a few days) finding them somewhere else to stay that suits their needs may be the best option.
To protect yourself against Airbnb refunding them, make sure all they have said and all you have done to address their complaints is documented in messages on Airbnb.
Good luck!