Some mobs, like Anantaboga, are immune to stun.

For other mobs, shield bash (initial hit) will stun the mob for 6 seconds to allow you to wail on it unfettered.

Just like other stun skills, the enemy begins to build up stun resistances over time.

Take Ifrit, for example.

It is reasonable to see 6-8 eruptions before Hellfire, and if lucky none of them get resisted. It is however possible for #6, 7 and 8 to be resisted, as you have used it so many times before.

One point to this- Only the PLD should ever be stunning, if anyone else lands a stun in the fight, it WILL contribute to building the enemy's resistances.

Hellfire does reset his resistances, though, so you can have a fresh go at it afterwards.