Too short of notice to lose any real fat, and it's a bit pointless to diet for an event (unless you're committed to the changes made the weight usually piles back on, very quickly, and with a few additional pounds too!).
I'd recommend shapewear instead - given the cost of the dress a nice corset wouldn't be an unreasonable spend, especially a waist cincher (they're very inexpensive to construct, as far as corsets go). That will give you the silhouette without a crash diet.
If you must diet, go low carb until after the event. That will cut out water weight, bloat, and should burn a fair bit of fat, too. But unless you continue after prom it WILL spring right back on with resuming your current intake. That's why I recommend it far less than just using a garment to get your belly tight.
Other solutions would be heavy weight lifting, in general. But a month is too short a time (by far!) to see any big results and the real benefit of lifting is over the long term. Maintain a regimen with heavy weight and after six months or a year the differences in your shape will be remarkable. If you already work out, as your post indicates, then I'd highly recommend focusing on strength as opposed to cardio if a tight body is your desire.
A month, though, is really too little time for any reasonable solution (other than buying a dress that fits and flatters your body as it is right now
).
Prom in a month, need to slim:)
