Extreme Difficulty is under Extras > Goofball. Initially you needed to read the story normally to unlock goofball, but since the remaster this is not required.
There is a special command, while in Goofball + Extreme, that'll show up in Help that'll slowly fade in. This special command can be used on the wait timers and will not invalidate the achievement. There is a catch though: You have to use it on ALL TIMERS. This may take around 5-10 minutes at the quickest if you're skipping the text.
When you get an orange pop-up at the end of the story scolding you for skipping, the achievement should trigger.
Disclosure: I used skip text when getting this achievement as I read the story years ago before it got remastered, but due to save files being incompatible I also had unseen text checked. I cannot verify if the orange pop-up box will still show up if you only skip the timers and/or if the orange pop-up is required to trigger the achievement. Some verification will be needed.
Another method I discovered by chance, however tedious, redundant, unnecessary and don't recommend, is save at a timer, exit to title, load the save and it'll skip the timer completely, however I cannot verify if this will invalidate the achievement, but at the very least will take longer.