Some fellow Steam gamers have complained about this sequence, saying that it's too difficult. Actually, it's a lot easier than people seem to think and there is nothing random about it: 27 blue clicks (left mouse button / Xbox controller: X) and 12 yellow clicks (right mouse button / Xbox controller: Y) when they are needed and the job is done. The times you get hit and injured by the enemy are unavoidable, you cannot stop that from happening. But as long as you keep to the colours all the way through, and don't click or press anything else, you will survive the fight every time, no matter on what difficulty you are playing.
Note that Marius is severely injured at the start of this sequence - after all, he has just been dealt a very serious injury by Nero. He doesn't have a lot of health and is practically at death's door. But by following the colour prompts on the screen and responding with the correct mouse button he picks up little bits of health, enough to get him through the fight in spite of the injuries he suffers along the way.

ADDED LATER: NOT SEEING THE COLOUR PROMPTS?
∙ It has come to my attention that many players don't actually get the colour prompts in this fight. I totally understand why that would be a big issue and why the affected players would have so much trouble finishing the sequence, thinking that it's all random. Hopefully the colour-coded chart below will assist in that case.
∙ Possible sollution supplied by a player called Multiple Cats:
Try reducing or turning off supersampling if you don't see the colour prompts in this sequence or boss fights in general. "That worked for me."
Thank you, Multiple Cats!
Please let me know if this helped you.

NOTE ON THE VIDEO:
Since the question came up, I'll explain. I am not getting legendary hits in this video because it's a help video. I was deliberately delaying my hits to show the colour prompts clearly. The video was never meant to show off my gaming abilities. It only shows how straightforward the fight sequence actually is. It's not about reacting as quickly as you can - you can do soldier hits from start to finish and still make it through. It is about getting the blue and yellow prompts right.

Video (2:57)



Only blue and yellow prompts are involved here.
You take NO other action:
No green or red prompts. You don't use the focus and execute keys. You don't even use the movement keys - the whole sequence is a programmed animation.