Discovered by AKA, Kazooie

Only Shadow and Spirit medallions are required to get Light Arrows. The reason for this is that the game normally requires forest, fire and water medalions to even enter Shadow temple. So it assumes if you have shadow then you must have forest, fire and water too, but thanks to Shadow temple early you can skip those 3 and get into shadow whenever you want. Spirit is required because it can be gotten at any time even under normal game circumstances.