Any battle, especially a dangerous or important one, always had room for wounds, often quite serious or fatal. Sometimes, a single successful attack could end a fight as a sword pierced an opponent's heart or severed a head. Well, these were extreme examples.
An experienced fighter fighting an opponent of similar strength would never let the fight end that way. So, most often the outcome was other wounds and the fight between the Progenitors was no exception.
On the contrary, when Sven fought Polosa, the Queen of Plants received dozens of wounds, much more than Sven because she possessed regeneration. When Alucard fought... well... for him, getting wounded was a real fighting style that only he alone had mastered perfectly.
Everyone had different regeneration as well as defense, so different opponents needed different damage to reach the critical point.
In Nitra's case, she had already reached her limit.
