Serious Question : Nerscylla Check (Savage)

Am I missing something? I hit one with 4 cactuar, it got paralyzed too, and I use a 500 Fire elemental damage on it without stopping my combos or getting hit. Why is it literally never working? (Most of my skills are turned towards offensive since I play with Support Hunters)

I have got it once out of maybe 15 attempts today, and ended up dying because Omega stunlocked me afterwards