Its nothing personal GreninjaBisharpSWAG, its just that this moveset isn't very well thought through:
Darmanitan has a very high physical attack stat, however his special attack is terrible; at base 30 it won't be doing any serious damage at all. while both flare blitz and superpower are great and valid moves, solar beam is an ill-thought out choice for darmanitan, as not only would it take 2 turns to set up (unless in sunlight, which isn't stated in this build), it would be an extremely weak attack, due to solarbeam being a special attack. another note is that Darmanitan cannot learn recover at all, not even through breeding, so people reading this moveset would be left annoyed at that fact (although personally, if darmanitan could learn it then it would be amazing). darmanitan can however learn rest & sleep talk, which could possibly replace your recover & solarbeam yet still allow darmanitan to recover his health.
If you ever want to make another moveset for a pokemon just quickly check the list of moves it can learn on the previous page so that what you write is valid and don't end up disappointing anyone.
Hope this helps