Hello! I'm really glad that you liked the game. The fifth circle can be a little confusing, but it has an end. Sometimes Subject 61 will dash towards the player and unleash a shockwave. After each hit, he becomes angrier (after all, you dodged). After 3 hits, he will speed up 2 times, and on 5 hits he will break through the floor, thus winning will be counted (don't jump into this hole :3)
On this circle, you must always be on the move to run away from the boss. If the situation is critical, use a dash (for example, a stone flies straight at the player). Moving quickly in a circle around the boss will cause him to miss (but you need to be extremely careful when he speeding up). I believe that you will be able to defeat the subject. After his defeat, talk to Belphegor and you will hear pleasant music.
Have a good day and mood!