@Mogima
Being a woman, I can assure you that the monthlies are at worst "annoying, painful and irritating". It doesn't make someone a cripple. Even a woman with the worst period pains will still be able to do their jobs.
So unless this is the kind of "OMG, KILL ME! KILL ME NOW!" kind of fantasy pain where you'd rather commit suicide every month than to go through the monthlies, her fighting skills would not be affected much. But to be generous, let's just say that her power was reduced by half. So her half could not defeat a full beastman power. Then he beastified, which makes him go maybe double his power. So hypothetically, she can still defeat the beastified him when she's at full power.
But now, he's cursed in a way that even the protagonist has to get serious. But the protagonist is the most powerful person in the world, which she couldn't compare. Yet he finds this troublesome and has to decide whether he wants to kill the guy or not. If we want to be generous and say that the protagonist is only 10 times stronger than her (which is not, but let's be generous) and he finds this difficult that he has to resort to killing, what exactly can she do other than die?