The armor can be upgraded by a man to the north of Artisan's Way near the Arena after Chapter 22. After it is +6, the armor will require Ancient Alloy to be upgraded to +9, which can be obtained by killing the Berserk Kraken or stealing one from Beserk Kraken. These are the only two ways to obtain those in one playthrough. When you're first place in Deathmatch, there is the possibility to gain Ancient Alloy too. When upgraded, it gives 100% chance of being resistant to knockback.