Note: Pre-Release versions of this game have 2 glitched achievements so far, this is one of them.
Crystal Sage is the 4th boss you will face and is generally straight foward but you can be killed very easily if you don't focus. When you first enter you can run up and hurt him and he will disappear but don't fall for this act as crystals will spawn around the arena and must be destroyed by running into them or more Sages will spawn and when multiple shoot you it can be very deadly.
The more damage that gets done to Sage the more crystals will spawn so at this point late in the fight just keep running for the main sage and after killing him the rest will go away.
Below is a video from powerpyx: