So I have did little tests here and there to test if this stigma is PvE and it appears that it is. I have only tested physical attacks so far.

How the test was performed for physical attacks. Both me and the mob did not move for the recorded damage. I did not count parry, block, and so on. I did not have any buffs on but I was in my bastion the whole time testing. Debilitating Blade VI was applied for the damage reduce approximation test and only hits that were for sure done while the effect was in place were counted. I only did a small trial of 20 hits for each measure of no debuff and debuff applied from a normal Level 58 mob (my Aethertech is Level 62).

I didn't save my logged hits after the test, but I did however saved that it come back with that attacks were reduced by about 30% in damage when Debilitating Blade was applied.

I also noticed that some attacks from very low level mobs that hit around 300 damage had their attacks reduced to 1 damage hits. I am aware of how attack works so mobs that hit 1000's will be reduced less in percent than mobs that hit 500's.

It is my guess that the ~30% less damage number will hold for any normal mob at any level with the current version of the stigma (as long was that stigma exists at that level). It is also my guess that bosses hit approximately up to double damage than mobs so the result on them might be ~15% less damage when Debilitating Blade is applied.

Gladiators have a skill called Howl which is better than Debilitating Blade since it reduces attack by more and that it is an AoE. Gladiators can probably see great uses in this if it fits in a stigma build and if the instance has strong adds that are a great pain because of their damage. I will also like to add that Songweavers in 4.8 get a stigma like Aethertechs with a smaller cooldown, but approximately the same in duration and debuff.

My next tests will be for if a mob or boss has Magic Boost and how much damage can be reduced with the stigma if the typical mob or boss is using Magic Boost. I was thinking if someone wants to test Hyperions 3 AoEs at the beginning. 1 AoE no debuff, 2nd AoE debuffed, 3rd AoE debuffed and report results, but it doesn't seem like the mobs follow normal game mechanics.