lol, i tried out the Poncho and Pointed Hat against the final boss--yup, a defensive Sun affinity of S nullifies the spells completely. i made Lyon's Sun defense S-ranked with 1 of those 2 items and gave her a Steel Rune to greatly reduce the physical damage. the result creates a truly broken combination. with these pieces, it lets you get as creative as you want the rest of the equipment and rune slots. because these are the only 2 Sun defense equipment in the game, only characters with a B or higher in Sun affinity can move up to S, of course. keep in mind that the physical attacks are also limited to the front rows, so you can skip the Steel Rune with the right formation/order.
Characters with natural A in Sun: Prince, Lyon
Characters with natural B in Sun: Alhazred, Bernadette, Cornelio, Eresh, Isato, Jeane, Josephine, Kisara, Kyle, Lelei, Levi, Lorelai, Nelis, Norma, Sharmista, (Sialeeds), Urda, Zerase