How is stun (resist) work in general?

[Azoryl]

I'm leveling my PLD and running some early dungeons. Yesterday I was doing Hakku Manor (lv28 dungeon). The final BOSS has 2 abilities with long casting time. One AOE and another with high damage. I was trying to interrupt them every time but find myself failing quite often lately.

Thanks.

Just break LOS (by running behind her) on the Void cast and the Fira cast.... A DPS could tank her because of this.