Warden's Sonic Boom is technically an air element attack since sound is vibrations in the air. In vanilla the attack can only be reduced by the Resistance potion effect.
What if Spell Reflection enchantment from a shield could bounce or muffle the attack? If bouncing sounds too easy, muffling wouldn't be too op as it could just be a 25% reduction per level allowing to only block 75% of the attack.
Or maybe it could be reduced by the Air Element Focus.