I think I have finally broken through the invisible ceiling yesterday - friend helped me farm Nergigante a few times, so I was able to make a few pieces of high end game armor (Dragonking patch, 2x Dober, 1x Nergi) as well as Nergi’s Charged Blade, which made all the difference in the end. Sadly I realised I’m not good enough (yet, anyway) to play without a shield so damage wise this was the best solution. In any case - took down Black Diablos (investigation this time) again and I only carted once!
edit: I will say this though, trying out many different weapons made me a lot better at handling the CB. Greatsword in particular was a great learning experience on how to time attacks, which is a huge help when you want to unload the super amped elemental discharge on the monster’s head.