Because your stun is on the global cooldown, it is nearly impossible to stun every erruption, you'll usually stun it too late. in end game 8 man raids your offtank will usually be dedicated entirely to stunning the boss. In addition, you can stun ifrit 3 times before he becomes resistant to it and will no longer be interrupt-able. In ifrit hard mode, his resistance resets every time he uses his charge move so it's not a big deal, I'm not sure what the case is for ifrit normal mode. This is important however because you need to make sure that your other classes aren't trying to stun the boss as well, other wise he'll take 3 stuns on his first errupt and then will be immune until his resist resets.