i'm wearing the mirror knight armor (not the helmet, gauntlets or leggings) and the desert sorceress skirt; i enhanced the armor to +5 so its physical attribute is 313; i assumed i would be an unstoppable force; however, i notice that upon exploring the dragon citadel (or whatever that place is) and the shulva dlc, my character takes as much damage as if she had just a regular armor set on; why is this? I spend thousands of souls and titanite to upgrade the damn thing and when i get hit i take 1/3 to 1/2 my bar off

Physical defence is MUCH weaker than any kind of elemental defence. The difference between 100 and 300 physical defence is about 40 points of damage. That's negligible given the fact that you should have 1200+ hp.

Well its been a while since I've played DS2 but iirc blunt weapons are good against armor so if you've been facing enemies that have weapons that do strike damage (basically any mace, great hammer or great club type of weapon) it won't help you that much if you have a lot of armor on. Also, there are better armors in the game (like havel's) that will gave you better defense than the looking knight armor. Also keep in mind that armor scales with this game so the higher your natural physical defense is (which is increased by vit and a few other stats), the better the armor will be. DLC is also much harder than anything in the main game so its not surprising that they are hitting your harder because the dlc enemies hit harder and have more resistances than anything in the main game. Anyway, hopes this helps :).

Honestly every time I play through this game I wear no armor at all. I don't know why people feel the need to turtle up so much.

The only reason to wear armor in PvE stuff is to be the prettiest.

They absolutely destroyed armor between DS and DS2. My DS character had a set of fully upgraded Black Iron Armor and I was unstoppable. Literally, nothing phased me, I rarely had to even slow down. I'd dash around, pause just long enough to heavy attack with my bastard sword, and keep running. Artorias was the first real challenge I met after upgrading that equipment.

In DS2, I upgraded my go-to set entirely before realizing that it does next to nothing there. As I got better apparel, I avoided upgrading it. On one hand, it forces more methodical play. On the other, it frustratingly dissolved a game facet I enjoy: equipment management.

EDIT: If I recall correctly, my character's currently wearing the box art armor. When fashion is all that matters, it's hard to beat that fur collar...

it's all about fashion, gotta look your best when fighting bosses.