hmm his skill set doesnt seem like it fit a Heavy Knight lmao
increasing your attack and defense when below X% HP and having a skill that make your defense 0 while increasing your attack by how much def you're missing seems like its a dominantly Berserker-type skill
if it were me, i would give him a Body Slam type of attack where the damage is calculated not from ATK power but from his Def, call it Shield Slam or something instead of that 0 defense skill. That would fit more with his class and its not a double edge sword skill that's more favoured by Berserker classes.
Imagine him stacking all that defense skill bringing his DEF to like 1000 (500 with skill + item stack and double it when below 20% HP) and just release the Shield Slam skill and just OHKO bosses. Still works with how he fight and make sense thematically
But hey that just the D&D Player/Dungeon Master in me talking.
edit: Luce skillset is thematically and synergistically perfect, so kudos for the author for that.
Because it isn't, we literally saw an entire arc revolving around him earning money in order to buy the skill book needed to attain that ability. That skill was never meant to be used by the heavy knight, at least not naturally.
Think of it like a TM from Pokémon, as in anybody can learn it, so as long as you have the corresponding item for it.
The entire reason that skill became so broken was because of how it interacted with the heavy knight; with every other class it worked as intended, it just so happened that the devs overlooked the one class that had insane base defense and broke the skill into the OP monstrosity that we just saw.
Though your proposed skill does seem like a nice idea (speaking as another D&D player), maybe we will see it or something similar to it later on as an acquired ability from leveling his other innate skills.