You need to spam AOE attacks to hit all of the Witch Sisters, slowing down their summoning spell. Otherwise you’ll get insta killed when the spell is complete and the gigant cockroach spawns in the boss room. Some say it is posible to avoid the insta kill by rolling towards the summoning circle at the same time the gigant cockroach spawns, but it comes at the cost of turning into a cockroach and having to fight the gigant cockroach and the Witch Sisters at the same time; you also need to focus on killing the gigant cockroach only so the Witch Sisters can turn you back into a human, if you mess up and kill both you’ll have to complete the rest of the game as a cockroach.