If you want more invincibility frames rather than attack damage, get the cloudy glasses and torn notebook from gerson. If you want attack at the expense of i-frames, use the old tutu and ballet shoes. Put all the other weapon/armour items in the box or discard them. For food, stow away Toriel's pie in a box for later. Have the quiche in your inventory, everything else is cinna buns. For the fight, just start to memorize what attack is when. If you know you can survive the next volley, then attack. If you think you won't survive, then heal. Try to heal sparingly, because you only have 8 items.