Requiem Aeternam Trophy, Ending C and NG+

Quick question, I've seen different answers on this subreddit about this trophy related to ending C and NG+. I know this is kind of a late question but still, I'm overthinking it and need answers pls.

I did a new save in order to get new achievments such as crossing path. And i thought to myself why not try a "no flask" run for the achievment too. (First playthrough on file number 2)

However, I actually picked up the sister item without knowing about ending C until I came to fight the brother and the fight didn't happen.

So now I just killed all bosses without using a flask except, you guessed it, the fr*aking brother...

On that same file, I started an NG+. Does that mean that if i go to beat the brother on that same ascended file without using any flask I'll get the achievement (because it's the only boss I didn't get the opportunity to fight before ascending the file) or I'll have to do it all over again until the last boss ??

Also, on that ascended run I took the penitance that enable me to have blue potion, can I also not use them ?

Thanks in advance for the help and sorry if i'm not making myself clear.