How is stun (resist) work in general?

[Raminax]

Stuns, and by extension, most if not all CC suffer from "diminishing returns", i.e. the CC becomes less effective the more it's used within a specific timespan on the same target. Most diminishing returns have a timer of 1 minute from the start of the first applied CC before they begin having full effect again, usually going 100% > 50% > 25% > Resist. Let's take Shield Bash as an example, which lasts 6 sec baseline with the trait.

Stun at 0:10 - 6 seconds (100% power)
Stun at 0:30 - 3 seconds (50% power)
Stun at 0:40 - 1.5 seconds (25% power)
Stun at 0:45 - 0 seconds (Immune/Resist)
Stun at 1:12 - 6 seconds (100% power)

And so on. Diminishing returns always reset 1 minute after the first CC is applied, regardless of how many times it's used. Ergo:

Stun at 0:10 - 6 seconds (100% power)
Stun at 1:20 - 6 seconds (100% power)

As for Ifrit, he does use Eruption quite often during the first two phases, but usually not enough that you'll get Stuns resisted. You will get a Stun resist if you have bad timing on the Phase 1 to Phase 2 transition, though (Ifrit doing Eruption and dropping below 75% within seconds of it).