he did defeat the homunculus three times, but the problem is that, that creature literally rewrites what have happened, there is only a microscopic time window to be completely destroyed, romantica did hit him in that window and cause him to recover slowly but she did not kill it, and remember that desir is fighting in disadvantage, he is trying to buy time for all the civilians, fighting in a small place and against a fast melee combatant being a mid range suppor or mid range bust mage,