It should be possible to defeat her in a head to head battle in the final version of the game. But right now the available fights to gain EXP and Stamina are pretty limited, so you have to rely on the Ero Attack method to wear her down and yeah it is still close. She's either the strongest or second strongest character that the player is currently capable of fighting, so I think the difficulty level is adequate.
If you go by the given flavor text, Raptor suggests that he would have no problem fighting the entire Bellute army, but if he had to fight Garnet at the same time it could become a problem. This suggests she is stronger than the entire combined might of the army on the continent. Possibly including Merlin as well, which is why I say 1st or 2nd strongest.
Merlin has less raw power than Raptor, Polyquin, or Garnet, but he's also less of a direct attacker and more of a plan-to-win type of guy. Polyquin has a lot of power but her personality holds her back and she ends up being pretty ineffective. Garnet is the remaining one who is highly powerful, doesn't rely on magic, and even has the magical resistance to power through a lot of spells... and she doesn't play around and can actually put up a fight.
It will probably end up being one of the 3 most difficult fights in the entire game when all is said and done (I consider Merlin to be competing for 4th)