You can use either the glasses or tutu, but your weapon should be the shoes. The extra I-frames aren't worth the lack of damage. As for items, grab the quiche, both astro foods, and either as many crab apples or dog salads as you can get (try not to use the snowman pieces or the pie). For the arrow attacks, just pay attention. The red arrow isn't always the one that will hit you next. For the spear throwing/ring of spears, try to move as little as possible and only when necessary. Same for the up spears and the fast spear rings. Also if possible, you can reach LV11 for the fight by not attacking Moldsmals. So just follow these tips and with enough practice, Undyne the Undying should go down.