umm speaking as a 4 time genocidal murderer, take the attacks one by one but not in a sim. each death, try to prolong your existence by one attack. also, go into it with sea tea, as you are going to need that for his final attack. also have the frying pan, because it heals you extra when you eat an item and otherwise makes no difference to the fight
dont use a sim because those are often different from the actual fight, or the battle system is different, or the hitboxes are very large/small.