You love it, you hate it, it doesn’t matter this is monoalphabetic.
It was clear that it’s just a monoalphabetic cipher or fixed substitution cipher - so every letter is replaced by another one based on a fix alphabet.
I’ve never broken one of those before, but they are not hard if the ciphertext is long enough - and this one is.
I’ve worked in a way that every decoded letter is uppercase, it was easier for me to distinguish them from unknown ones.
One mistake I’ve nearly made was search-replacing - but that might have replaced already-replaced letters, so I’ve wrote a python script that goes letter by letter.
I’ve ran a few statistics on it and calculated letter frequencies. Based on the english letter frequencies I’ve found that
E. Based on that and the 3-letter groups I’ve also identified
I’ve also found
Next I’ve guessed
A from frequency, and found
sAST and guessed it as
L - also
SHApES HIS HEAa - clearly meant to be
shakes his head.
I’ve continued to guess letters based on words, until I’ve had a full decoded sentence and could google that - and found that I’m decoding
Mad Max 2: The Road Warrior’s script (link).
Based on that, I was able to decode ALL letters and get the flag inserted in the document.