So, it seems that Siege Warfare must be the best place to prove your ability.

Go to Tiamaranta, and join four bouts of siege warfare, one for each of Tiamat's Incarnations.

If you prove your worth and come back, I'll help you extend your Greater Stigma Slot through a Mysterious Stigma.

Player: Accept.

Beautiful, Player! If you managed all four bouts of siege warfare, you're more than capable of handling Greater Stigma.

I'll give you a Mysterious Stigma, so please hold it in your arms for a while, until it has absorbed your energy.