Safer Phase 1

As it currently stands, there are ways that Phase 1 may become permanent. For example, if a player abandons the battle and the Drones are either despawned, taken away with mob transport, or unsafely removed. Proposed fixes include making Malachite Drones refuse to despawn if an "owner" is set, decrease tally if a drone is despawned, starting Phase 2, potentially have a failsafe removal if an entity is taken too far away from the Drone for too long.

Height-based Damage Resistance

Due to pathing AI in Minecraft being poor, the Malachite Guard cannot hit a player hiding in the stairs. The idea is that this cheese can only happen on Easy mode. Normal mode will halve the damage taken, and Hard mode will have total immunity. Height immunity kicks in if the player is either 1 or 2 blocks below or above the Malachite Guard.

Disengagement Reduces Damage Taken

Say a player decides to stay a long distance so the Malachite Guard cannot target them but the player can still hit it in some form. In this case, the Malachite Guard will take reduced damage (Normal) or no damage (Hard).
In the event of multiplayer, if the Malachite Guard is engaging with a target, reduce damage from attackers by half (Normal) or a maximum of only 2 damage (Hard). Perhaps also give it the ability to switch targets with a cooldown timer for when it can switch, optional "rage" mode that makes it faster.

(Consideration) Ranged or Area Attacks

Attacking only one target leaves much to be desired for a boss. If anything, it acts more like a glorified mob entity. This is only a consideration as the scope for a ranged or AoE attack is a little too far, not unless it is brought in enough to work for the Malachite Guard.

(Consideration) Natural Spawn Prevention or Extra Damage to Non-player Entities

There is one problem with two solutions. The problem: Malachite Guard can be distracted by non-player entities, such as Crystal Golems that may spawn naturally on the structure, or pets.

One solution is to prevent natural mob spawns in that area. It seems unusual that non-hostile mobs seem to be able to spawn on top of the structure, worse so when they are mobs that may target hostile mobs like the boss itself.

The other solution is to make the Malachite Guard ignore them entirely. Since Normal and Hard mode Phase 3 makes the Malachite Guard ignore all damage taken from entities that are not an instance of PlayerEntity, it may make sense for the Malachite Guard to also ignore entities during this phase, perhaps also in Hard mode, it will ignore non-player entities in general, Normal mode will ignore on Phase 3. Knockback is also ignored in Phase 3, so it is always worth trying this out. Perhaps also an effect and nullify all effects of an attack if the Guard take immune damage.

This has been successful. No need to keep this open anymore.