Obviously, something for all the optional bosses. Ohhhh there's a lot of them. Since it's easytype, winning some of the tougher ones without using certain abusable items (invincibility) might be a good choice as well. Granted, some are tough as nails even with those, so probably not for everything, but still.
Avoid 'X number of jumps with marios skill'. There's an item that more or less lets that go perfect, so there's little point unless you can find a way to exclude it.
Doing something out of order might be a good one. Beating X hidden superboss before beating the game (thus before accessing some of the best gear) is a possibiliyt. Making it some (or all) the way through the icy pipe sequence before X storyline event might be another. (It's way beyond an average leveled party even post-bowyer when you first arrive at it.)
Maybe something for the triple-timed weapons. Those are new in the hack (no attack had more then 1 timed hit in vanilla, so using an achievement to point players at a triple-timed attack at least indicates they exist to them. Not everyone's gonna read the manual beforehand.)
Boss rush is a thing, deserves some recognition. Maybe even a turn count leaderboard or something. Not sure how feasible that is.
Maybe theme some achievement names off other SE franchises, since so many bonus bosses are tributes to various FF games, following that theme with FF tribute achievement names (not necessarily just for those bosses) would be a wonderful fit.