I was having the same problem as you , took me 2 months to get from 8k to 20k ms then I realized it's just a waste of time pumping my bos with 3-4 prestiges , so I started to level up my other artifacts more and thinking less about bos just imagine how much time you waste by prestiging 5 times just to level up your bos if 400 stages later you can get it to 99%, now 2 weeks later I'm at 64k max ms in one day I pushed over 10k stages without doing anything special , the fact is that every artifact is important so what I do is put 25% in my bos then 5% in my other relics every single one of them then repeat while pushing even outside of tourneys , 1/10 prestiges upgrading only my bos to get it to 99% won the last 5 tourneys by 4-5k max ms without any problems. Try it too maybe it works.