Jayson combat inflation

after passing 9 waves out of 10 waves, boss Jayson appeared

Jason is a unique boss because most of his attacks can be parried at all times. One parry deals 525 damage and heals you for 52 health, making the Sword essential for taking him down quickly.
For Jason in particular, managing multiple parries relies heavily on your positioning to be successful, especially when he is enraged; after parrying Jason to get him into an enraged state, focus your attention on his body posture and pay attention to his hands; when it starts moving, it means he is about to come out of his stunned state and you should get ready to swing your sword. Immediately after parrying, quickly dash back to get into position for the next parry. This is easiest to do in a solo run, as Jason is known to often switch to nearby targets when there are many players around.
The ideal trajectory for Jason to follow you is a straight line. Any other direction to the side can have varying distances, often making it too short or too far away relative to a single dash. If you still have 2 white bars left, you can interrupt this attempt by doing a dash jump away from him. Try to focus on letting the white bar recover, but don't be too conservative with this, as you may need to dodge Jason's attacks once you're in this scenario.
You are out of luck if you have 1 or 0 active white bars. You have no choice but to tank the hit, however if you are low on health you can die as a warrior and go to Valhalla trying to parry in the weakened state you are in since you don't have many options other than just dying. You still have the opportunity to regain some health since parrying does heal a certain amount, so not all paths from here lead to a bad end.
The miss was parried.