All I'm putting forward is that that cap might be able to be reached on most enemies with less skill than people assume.
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I myself have tanked without Shield Skill assist gear for the last 4 years. I've even fought and survived such enemies as Alexander with little descernable difference to documented performances of other PLDs who have attempted the same fights.
I have however NOT tanked a Sky HNM or any others of that nature except for the Nemain Lion in Besieged Battles, and the I will admit was done mostly with the generous and strategic use of temporary items, but I assume it would be the same for anyone other PLD attempting to fight that nasty.