Yeah, theseus/asterius has me stuck for a while too. There are two required things to keep an eye on for the fight, and then it's very doable, I never lost again after these fixes:
As soon as you see that crosshair on you, assume it's already been on you for a few seconds. So start moving perpendicular to dodge. He doesn't predict where you're moving, so as long as you're moving perpendicular to his aim, he'll miss. Don't dodge back and forth or something.
If Asterius stops moving for a second, he's gonna do his two big swings and then his jump attack. Those two big swings are the most damaging thing in the whole fight. So if he ever stops moving, get the heck away. If you're a pro, dash through him and get behind him. After the two swings, be ready for the jump attack--it's probably best if he jumps over you and past you.
And of course, kill asterius first, before theseus enters his 50% mode.
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