Crystal Guardian

First Encounter
This is a difficult boss to defeat, mostly due to difficulty in finding a safe spot. The best strategy is to slash at the Guardian a few times and then get distance to avoid the beam from its arms. The beams from above have very little warning as to where they appear, and have relatively small gaps between them. As such, it's actually usually safer to simply walk to the open gaps rather than dash, as there is a risk of overshooting the safe zone and dashing into another beam.
 * Beam: Crystal Guardian will shoot a beam of crystal energy from their hand. They target The Knight with the beam, firing it where The Knight was when they started their attack. The beam is dangerous for only about half a second.
 * Sky: Multiple beams of crystal energy will simultaneously shoot down from the sky. These beams become dangerous at different times and each beam stays dangerous for about half a second. These beams will appear throughout the arena. Each beam can be at a different angle, though, the angle will always be between plus and minus 15 degrees of vertical. This attack can happen in conjunction with Crystal Guardian's Beam attack.

Second Encounter
He appears in the room above the bench where he was first defeated. He has the same attacks as before but is more aggressive; his attacks deal two masks of damage instead of one; and the Sky attack beams appear one after the other rather than simultaneously.

There are no safe spots in the boss room, but when standing on the far right of the room, most of the beams from above won't hit the player, so they only really have to watch for the Guardian's Crystal Beams. When he jumps close, he tends to shoot backwards so it is easy to land 3-5 hits each time.

Trivia
Кристаллический страж
 * The first time the Crystal Guardian is encountered, it's seen sitting on a Bench. It only engages in combat once attacked.
 * The Crystal Guardian's battle theme is the Crystal Peak theme with an added instrumental layer.
 * If the player dies during the second fight with the Crystal Guardian, their Shade will appear in the boss room, and they'll likely have to fight the Shade and the boss at the same time.
 * It is possible to draw the Shade out of the room and avoid getting locked in by staying close to the doorways and jumping to get within the Shade's engagement range or using Vengeful Spirit.
 * Along with Hornet, the Crystal Guardian is one of few bosses that is fought twice.