TL;DR
Give defensive armor a special near death health that only protects your actual health from their corresponding damage resistances that activate upon reaching 3 bars of health or less and lasts per life.

Just to make things clear, we are considering all pure defensive and no offensive buff armors as a defensive armor. This does include the Mercurial set even though it provides movement speed because movement speed could be both offensive and defensive.
I made this list to show the current average for "defensive" armor in the game. As you can see from these numbers all break above 16.4 per piece while some reach over 22. This seems rather inconsistent.
Almirian Crusader Armor shadow:9~ normal:7.5~ Total:16.5~ curse resistance:1.5 fire resistance: -1.5 Total counting resistances: 16.5~
Azure Guardian Armor piercing: 9~ normal: 7.5~ Total:16.5~
(We are not counting the attack speed reduction for the plate mail lines due to that being an offensive debuff, not defensive)
Volcanic Plate Mail elemental: 7.5~ normal: 10 Total: 17.5~ fire resistance: 3 stun resistance: 3 sleep resistance: -3 Total counting resistances: 20.4~
Ironmight Plate Mail piercing: 7.5~ normal: 10 Total: 17.5~ stun resistance: 3 sleep resistance: -3 Total counting resistances: 17.5~
Dread Skelly Suit shadow: 9 normal: 7.5~ Total: 16.5~ poison resistance: 3 freeze resistance: 3 Total counting resistances: 22.5
Royal Jelly and Grey feather share the same total as Dread skelly.
Mercurial Mail piercing: 9 normal: 7.5~ Total: 16.5~ shock resistance: 3 movement speed bonus: 1 Total counting resistances: 20.5

So what the brown is going on here?
It seems the armors with the least debuffs have the most total defense!
How odd seeing how both the Alimiran Crusader and both plate mail lines have a debuff yet they share the same total damage resistance as the what I will call the "Best three", aka Dread Skelly, Royal Jelly, and Grey Feather.

So what am I suggesting here now that I have all this data? Well at first this was going to be a "last stand" sort of suggestion where if you have "number" amount of defense and a attack deals "less number" then you would not take damage if you are near death. Then I saw a few issues: UVs, and naturally low defensive armor stats.

So here is the new one. All of these listed sets should receive a sort of "Final Stand" health when they reach 3 full bars or less.

What are Final Stand health bars? They are health bars that protect actual health bars from damage correlating with their resistance. Blue bars protect you from normal, purple bars protect you from shadow, yellow from piercing, and green from elemental.
The average system will be Final Stand health= half of NATURAL damage resistance
We will be averaging all .5s to a full number and any thing bellow down.

List of Final Stand health per armor
Almirian Crusader Armor: 4 shadow, 4 normal.
Azure Guardian Armor: 4 piercing, 4 normal.
Volcanic Plate Mail: 4 elemental, 5 normal.
Ironmight Plate Mail: 4 piercing, 5 normal.
Dread Skelly Suit(Same stats for Royal jelly and Grey Owlet): 4 "armor designated resistance", 4 normal.
Mercurial Mail: 4 piercing, 4 normal.

"Now hold on a second.." Some of you maybe saying, while others may just be saying, "Oh fns get this over with already."

I didn't list the Ice Queen!
Your right I didn't. I think it is unbalanced. With how much higher its piercing is compared to the Royal Jelly set I don't think it should be listed due to imbalance.