To earn the "UNDEFEATED" trophy in Detroit: Become Human, you must make it through any QTE-based fight sequence without getting knocked out. I would recommend playing on the CASUAL difficulty setting to make this a bit easier, though the QTEs are generally fairly easy.
Each time a button prompt appears on screen it will either have a decreasing white line around the outer edge (press it before that line disappears) or you will be instructed to mash the button. In most cases QTE fight sections are a mixture of
button prompts but the game randomly throws some other prompts in as well. It's definitely possible to drop some QTE prompts and still come out of a fight OK but it's difficult to say which and when, so just try and hit them all... and avoid fights altogether where you can. If you fail a fight, you can restart the chapter.
Note that many of Connor's deaths aren't technically fights, so you can get away with a number of those and still get this trophy at the end. Just don't fail any QTE sequences outright and you should get this trophy when you reach the final flowcharts after the credits.