Unfortunately this article doesn’t provide much actual detail on the production process. From the descriptions here and watching the series, it seems to be what I’ll call “Borderlands on Steroids”. 3D camera, characters, and elements, but 2D matte paintings for backgrounds and hand-painted textures on everything to look pseudo-2D. The added 2D explosions and random elements are probably done in post. Great composition overall and very satisfying to watch.