Desire is a cunning opponent who will try to deceive you. During the fight, avoid her blows and wait for the second phase of the duel. When Usiel intervenes in the fight, you can ignore him. There's no point in him dying - focus on the main opponent.
First phase of the duel
In the first phase of the duel with Desire, focus on avoiding her blows and execute counterattacks. When your opponent's health drops to around 50%, Usiel will get involved in the fight.
Second phase of the duel
Now you have to fight with two opponents. Focus your efforts on the main enemy and avoid Usiel's attacks at the same time.