Fradz Quick Stab (-5 permanent armor, stacks up to -25)
Galeoth's Frozen Fang (-20 for 5 seconds)
Phodom's Meltdown (-25 for 10 seconds)
Doom Helmet Item (luck box max luck in chapter 6, -10 armor aura)
Urn of Chaos is supposed to reduce armor, but I don't know if it works (item purchase in chapter 6)
Santoryu item (from luck box forget which chapter) is supposed to reduce armor
Skull of Chaos (reward item chapter 6 from the Forgotten quest, I have no idea if this item actually works or not)