Do you think this would be a fun miniboss for a level 3 party as an ancient guardian knight of a storm tower? I'm going for the vibe of the dark nut fight from OoT where it gets faster and more dangerous as you cut off its armor. Its also going to have 3 or 4 Ice Mephit minions that I swap all cold damage to lightning.

>24 AC, 60 HP
>Speed 10ft
>Crumbling Armor: When an attack misses this creature its AC permanently reduces by 1 (to a minimum of 10). At 20, 15, and 10 AC this creature gets an additional attack and increases its speed by 10ft
>Innate Spellcasting (DC 12) 3/day: thunderwave (2d8 damage)
>Greatsword +6 to hit 2d6+3 damage

Am I overcooking it with the AC reduction on misses thing? Would it be better if I give it a lower starting AC and just say at certain HP thresholds its AC reduces? I kind of like this way because they dont necessarily have to get it to its most dangerous form if they roll well or the spellcasters focus on using saving throw based spells as those wont damage the armor.