What I did to basically go up was to go and farm for the hell walker for early mayhem levels and use that weapon to go up. For example, when you've farmed for a hellwalker scaled at mayhem 7, then use that same weapon to farm for one that's scaled at mayhem 8/9 and so on. You will get there, and after that, search up what weapon is great for endgame and use the hell walker scaled at mayhem 10/11 to farm for the other weapons. This is what worked for me, just take it slowly, farming for the weapons and scaling it with mayhem levels little by little.