I heard that you have to upgrade the model weapon, but I just can't see that as an option.

Does anyone have the definitive answer?

The Ultima Weapon starts out as a Toy Gun/Model Gun. 15000gil later it becomes the Silver Model Gun. 35000gil later it becomes the Gold Model Gun. And finally, 200000gil later it becomes the Ultima Weapon. I'm pretty sure you can upgrade this gun during a level, but if that's not working for you, try upgrading it between levels.

yes the answer is right there, i'm replaying the game in order to get the ultima weapon... i missed it first time around

So you need the money in order to upgrade? ie if you dont have the gill, it does not appear as an upgrade option?

I really don't remember Grimoire, I haven't played in a while. If you're short on gil, try the defeat 100 enemies mission in mission 9 I think. Each time you complete this, you should obtaim 40000 gil & the Golden Moogle Doll, which has no other purpose other than being sold for 37500.