So eventually you learn (i did at least) there are specific thralls that are just better, and i only get those. I just recently ,like really recently 3 days ago i think, abducted 2 relic hunters from Unknown City and 2 Cimmerian Berzerker from their usual spot. 3 of them are already level 10. These dudes come out of the box with 1500 to over 2000hp. they have on salvaged redeemer armor from the boss relic hunters from Unknown city that are just drops when you kill them. except for the Red Mother (and that damned snake cause the poison it does is crazy) any of my thralls can solo any of the skelly bosses and the brute too. I know this topic is about leveling and weak thralls, but thats the point. Dont get weak thralls, I can go fight the croc and spider world bosses with a level 0 berzerker in half broken redeemer armor as soon as i get him out of the wheel, and thats how you level them fast. Theyre boss killers at level 0!. So I know this may not be the solution you are looking for, but I think they designed the “power levels” of the thralls just fine in my opinion.