[EenKogNeeto]

Current Abilities:
Far off Shine
Summons the appearance of a random person as a Mirror Soldier who aids the Mirror Knight in battle.
Vanity
A true mirror's reflection copies every detail. Upon use, the user gains a copy of positive status(es) all enemies have, including the LV, duration, etc.
Mirror Armor
Grants a percentage of automatic retaliation of magic damage, based on level (max: 20%). Physical damage will shatter the mirrors for the rest of the round, preventing the retaliation effect, but will reduce the LUC the damager for the rest of the battle, as breaking mirrors is unlucky. - Bugged currently so physical damage doesn't prevent retaliation at all.

These are all kinda cool, though you only see this enemy in two places currently. With a bit of tweaking, it could be made a formidable boss in its own right and a good alternative to Ravenlord.

Non stat related changes suggested:

First: At the start of combat, has Spirit Mirror active and summons four reflection copies (Or just has them out at the start)

Second: Use far of shine more often (Currently summoned once over the course of a 40 round battle) - This should probably mostly behave however the crow lords does.

Third: Give it an alternative attack besides basic attacks to prevent cheese, shine ray would be fitting and already exists for an example. Perhaps it could attack with magic when its mirrors are up, and basic when they are broken or something similar to how the ravenlord spams magic attacks until it transforms, then starts mixing in basic attacks.

Obviously, it would need its stats buffed up to a point befitting a boss monster.

-Potential 10*: Mirror Plate, heavy armor which conveys the effect of Mirror Armor for UL/3 attacks (Retaliation for magical, LUC reduction for physical)

[Raigen.Convict]

Shadow Guardian is just fine where it is, now an alternate BDP which uses the Dark-manor mobs for its enemies would be good, though. Maybe a Haunted Manor BDP?

[WaifuApple]

The mirror knight boss seems more likely to come in a separate dungeon with the wasps and beetles that also currently do not appear in any dungeon, as far as I am aware. Rather than just throwing it where there's no point in it being, I'd advocate for some sort of temple BDP with this as the boss. One or two more new mobs to go alongside the two I'm aware of also from Bai Kai and you'd be set.

[Snake]

I agree with Apple. Not much to say here but that Mirror Knight seems to be a Bai Kai-style BDP thing.