As i said on mod DB, it should be something with mallorns. If not roots, leaves then. It can be strong wind, bringing them to wound enemies or something like arrow volley but with this golden leaves. (Once again sorry for my english, I'm not native speaker.)
I was thinking about a protective spell, rather than another one focused on violent disruption, with a wide radius on units and heroes, and solidly based on the lore
It is said that Galadriel, in the Third Age and long before the War of the Ring (almost 500 years before), created an enchanted white Mist with her Magic to temporarily protect Eorl, the ancestor of the Rohirrim, and his army from the Shadows of Dol Guldur.So, a possible ultimate power could give Galadriel the ability (via her Palantír) to summon this temporary magical Mist, that gradually heals all your units and the allies' ones in a wide radius, protects them from poison and fear, and slowly harms the enemies involved.
But Galadriel already has a holistic protective power, Nenya's Protection

So, another solution could be a spell that temporarily summons on the field a Wood (of Mirkwood, or a Wood of Golden Trees of Lothlórien), which grants the units and heroes on its area armour's and speed's enhancement, while slowing down the enemies on the other hand.