I've been watching some Mephiles runs and despite rewatching them a few times, I can't for the life of me figure out two crucial parts. What are people doing to make the Chaos Blob appear so quickly at the start during Mephiles' sword dash, and what are people doing to avoid the slow motion when preventing Mephiles' sword counter attack after you attack him five times in a row?

At the start of the fight, you have to double jump into him to activate the first phase of the fight. No other action will do it. The setup speedrunners use to double jump into him fast is-

1. Single jump
2. Spear glide
3. immediate double jump

See here:

Mephiles' sword counter attack comes out when you mash after the 5th teleport, so before the 5th teleport happens, hold the homing attack button.

See here: https://youtube.com/clip/UgkxD0CjqHJ28vgB-qRtC53R2y2RtZ5ruqi6?feature=shared