Evs-Win

A little late but just in case someone finds this... I did exactly this in UC4 in the Legacy of Thieves collection and it did work. I lowered the difficulty for Avery's House (Chapter 18), Ship Graveyard (Chapter 20) and the final battle (Chapter 22). After completing the main story I replayed those 3 chapters in chapter select and then went back to the main menu and selected play credits and the trophy unlocked. I managed to get through LL without needing to do it (though I came very close in a few places) so can't confirm but I would assume it would also work there.

After a few hours of repeatedly re-trying the trial, I just got it first time after watching this video. On my successful run I spent the entire level talking out loud to myself "don't rush, there's plenty of time". Previously I was taking way too many risks and dying needless deaths, especially in the back half of the level.