Hey gang, I found a video on YouTube talking about a strat to farm secret letters and, after trying it out, wanted to share it with you!

With this strat I was getting about fifteen secret letter clues ever 6-7 minutes (my highest out of six runs was 20), buffed slightly with Bro Boxie.

Basically what you want to do is start an exploration commission at a lower level (I was using lv. 30), start the commission and then stand over by the elevator. With Rebecca you just stand there with her E active and she passively one shots enemies that drop secret letter clues within loot pickup range. I usually went to stage 9 and then restarted since that's when the enemies got more tanky and required me to not afk lol.

Do this for an hour tho and (using my average), you could get about 150 secret letter clues in 1 hour - plus a bunch of actually useful ascension materials that you can forge into the rarer ones when you need them. Hope this helps!