Yes the % damage reduction is currently working and implemented. Max lvl Resist Elements is very good, not only it reduces elemental dmg by 40% (with divine might) but it also reduces the damage before defense subtractions.
So for example:
Let’s say a mob has 1.5k attack.
If you have 500 defense (which is quite easy to achieve with Aspersion buff from pardoner), it will hit you for 1.5k - 500 = 1k damage.
With lv 16 Resist Elements, you’d expect that 1k damage to become 600 damage instead (40% reduction). But that’s not what happens.
Instead, when you cast Resist Elements, you take 1.5k * 0.6 - 500 = 400 damage, which is effectively a 60% damage reduction instead.
Of course, the more damage the mob hits, the less your defense will matter. But Resist Elements can make your def / mdef become really important to reduce extra damage if we’re talking about hits of under ~3k damage. And remember it works for multi-hits too (such as spiral arrow), as long as they’re elemental.
I’ve been trying out Resist Elements + Healing Factor from PD (with Restoration Aura), and the results have been amazing so far. I’ve been able to tank ~5 mobs that hit me for 6k damage each (without resist elements) while standing still and not losing hp. I still have to test some more if Restoration really works with Healing Factor, but so far I strongly believe it does.