Desir is good at analyzing and deactivating spells.
Desir came from the future and knew possible outcomes, strategies, and information that he came across.
Desir goes into the past, before he was born, so he has a limited amount of info.
Desir is facing an unknown creature that doesn't use magic, has powerful physical abilities, can instantly recover, and is capable of analyzing opponents.
Of course Desir looks weak in this situation, I would be surprised if he was doing well.