Dosn't the manual explain in good detail how armor piercing works? Well, it should anyway.

It sucks when upgrading units online, if you just wanna increase a units defense you gotta upgrade armor, which in return upgrades armor and defense. If you wanna make a unit volar 0, but upgrade his defense and attack so when the unit gets a lot of kills it *if it gets a lot of kills*, it goes up in valor and has the weapon and armor upgrades. So, if you give a +3 to armor, not only will it tire the unit out, but armor piercing units get a bouns on the unit.

I wish you could upgrade defense without having to use armor upgrades.