Thanks for reaching out, it's a totally fair question! I'll try to put your mind at ease.
There's a couple things that will make this achievement pretty easy over the course of the game. First, I would recommend Chill mode, as the timer is much longer than Normal mode and as a result, more forgiving. There's no penalty for switching modes, you get the same rewards as you would Normal mode. Hit "B" on the title screen to change that in the options menu.
Next, over the course of the game you'll unlock gear that will make that achievement (loot), as well as others, easier to unlock. In fact, the very first item (pocket watch) is very helpful to keep the quick solves rolling. And keep in mind that a quick solve doesn't need to be flawless, so if you use up two of your guesses in short order to solve a code quickly, that's cool!
Truth be told, when I developed this game, as I got used to the puzzles I was worried it would be too easy. Hearing from a few who struggled early on opened my eyes to a few things that v.1.1 of the game addresses and hopefully makes things easier. As with any puzzle game, like sudoku, once you learn the patterns and what to look for, you may find yourself solving codes in 10 to 15 seconds. After all, every code is a 4 letter word, so it's kind of like Wordle, except fast (at least that's what I was going for). If you need any pointers along the way, don't hesitate to reach out!
As for the demo, I took 20 words and set (most) of them aside specifically for the demo, so it works just like the game. It's actually possible to unlock and purchase an item and unlock an achievement or two in the demo, but it's really a small flavor of the full game. Without the secrets, of course. The only difference is I haven't updated the demo to v.1.1 yet, so it doesn't have Chill mode in it.
Thanks again for taking interest in my game, I hope you have a great holiday!