I can attest to this. I used GTB card on a steel body champ tanking a ledged High Wizard MVP and was still taking massive damage from fire pillars, lord of vermillion etc. This situation im referring to was with the high wizard isolated via ice wall so none of her mobs were able to inflict any damage.
On a similar note, Staff of piercing/ Piercing Staff (ignores 10% of enemy mdef when performing magical attacks) currently is not working on assumptio'd mobs on lhz_dun03 (as well as other situations im sure). With 10% mdef bypass any wizard skills should deal 1 damage but they currently miss (no damage taken by mobs) altogether.