I have Abzi, and I'm tempted to use him, but he and Lucifur have the same problem of being all fire-based when the Hard level enemies are resistant. That, and Abzi is just extreeemely not my type lmao. I think I got Carlos but haven't leveled him, didn't feel like he offered enough to be worth all that time/effort.
Medium boss rush is definitely helping I think, still feels like I'm climbing a cliff but now making actual progress again. I guess that's kind of how the progress goes here? Up up up, wall, up up up, wall... It can work.