I believe with dials usopp can be near invincable. there are so many weapons he can build, I've designed a few:
Note: this is assuming Usopp has two or three dials of each kind, aside from impact, which he has more of and reject which he only has one of.
Dial shield: a large shield with a + of four impact dials and a reject dial in the middle. On the top two corners are fire dials, and the bottom two are breath dials. This could be used to block attacks from almost any weapon. A pure flame attack would get sucked into the fire dials, a bombs impact would get absorbed by the impact dials then the fire would get absorbed by the fire dials, and a smoke bomb would get absorbed by the breath dials. Because of its size (about two feet tall and one foot wide) it’s not a very fast defense. To block normal punched and kicks Usopp has an impact dial on each forearm. He refrains from using the reject dial because of the kickback from it.
Dial rifle: just a powerful tube, with a hole in the top near the back end. Directly behind that hole is an impact dial with an impact dial behind it. The tube bends down after the dials to a trigger that is attached to the first dial, when the trigger is pulled the dial goes off and the second dial absorbs the kickback. Also attached just in front of the hole there are four tubes with a dial at each end that forms an X like shape. One is a fire dial, one a breath dial, one an axe dial, and one a light dial. He can activate any of these for to add an extra element to his shot. He loads his ammo one at a time (or more) through the hole in the back. He still has all his old “stars” but can add odd certain kicks to them. Also it can be a flamethrower by turning on a breath dial and the fire dial at the same time. A small cover can go over the hole while doing this so that flames don’t come back at him. It takes longer to load the rifle, so unless he needs a very powerful shot he still uses his slingshot or kabuto.
Other dial attacks:
Sound dial: a recording of Usopp scratching a black board on a sound dial
I don't know where I got my dial amounts from, it just seems right. there is so much hidden potential in the dials and I just feltlike posting this