enemies & Characters become immune to Elemental Critical Hit after 5 hits. I discovered by accident that after some time passes, the immunity goes away and they take 5 more hits before becoming immune again. this continues until you give up or the enemy is dead. (I was using a low power character on a higher floor to see what would happen).
if you think about it, if you have 100%, enemies will be immune to Elemental Crit in the first 5 hits while they are still being affected by the first hit, if you have a high APS or Attack Frequency, or a low Cooldown.