First off, when you unlock the Neo-Geofront dungeon in Chapter 7, make sure to check every green message box on the world map as well as every new info source in Planeptune to get the tough foe recipe for the dungeon. Said tough foe recipe happens to require an item (Hell's Mane) that you’ll get automatically from fighting a boss in Thelad Sanctuary in Chapter 2, so you won’t have to worry about finding it unless you happened to sell it for some stupid reason. Otherwise, the method of finding it is ridiculous enough that you’d just be better off going for it in another playthrough.
Anyway, once you create the tough foe recipe for Neo-Geofront, you just have to fight the dragon tough foe (Tiamat) that takes residence in the dungeon. Hopefully, it will drop the God Soul necessary to create Nepgear. If it doesn’t, keep fighting it until it does, preferably going into easier fights in between to charge up your EXE Drive Gauge.
Once you have the necessary item, now you just need to get to Chapter 8 and get Planeptune’s shares up to 30% to unlock the recipe to create Nepgear from a message box on the world map. Once you do, you should already have the item needed to finish the recipe, so all you need is to get Planeptune’s shares up to 50% and be sure that you have 1024 MB on you to create Nepgear and get the trophy. Don’t worry if you don’t have enough MB; you can create Nepgear in NG+ without having to reach Chapter 8 again providing that you got the necessary item and you unlocked the recipe.