Watch at 2x Speed is recommended.
Anastasia has fairly middling hitcounts but she’s a Caster who acts three times per turn, with NP gain CE she just reached the threshold to reliably stack Asterios NP if she spends all moves on attacking (i.e. don’t use skills).
Then in an ideal scenario, Asterios should be able to solo without losing a drop of blood. Hence the L20 Asterios: with so little stat he could easily die in any turn where he could not reach debuff threshold, and the solo became even longer meaning more chances for things to go wrong.
This results in a gruelling 60 turn solo where every turn is a battle against RNG—for Anastasia to behave and charge Asterios back to 100%, and for Asterios’ attack debuff to not be blocked. If you wonder why Anastasia seemed so cooperative in the video, it is because every timeline where she did something to ruin the solo was pruned, just like her Lostbelt.
Anything you find strange like brave chain used or not using skill 1 can be attributed to a means of dealing with RNG.