Just the default difficulty. It's probably just a tiny bit harder than vanilla FireRed because there are a greater variety of Pokemon and I gave a lot of mid-game trainers better teams, but probably not that much more difficult. There are "super bosses" in the post-game though, all with custom IVs, EVs, items, moves, and abilities.
I have considered creating a "Hard Mode" branch, but that would be a TON of work fine-tuning all the trainer battles. I have another project in mind at the moment, so I don't know if I'm up to putting in all that work at the moment.