To be honest, the broken dagger bit I wouldn't find impossible to do and maybe even kinda easy but the "hard" part would be having the patience to do it.
I'd hate to do a challenge and only fail it due to dying of old age...
I initially played the game with friends too and while I kinda wanted to do it all solo right from the start I was kinda forced into a coop session with all of my friends with a lot of "No don't level up to far! No don't make too much progress lets all go together! Blah blah blah!" it was kinda annoying really.
Second time around tho I did that shit solo. Champion Gundyr and Nameless King both kicked my ass. Champion Gundyr is when I started to practice parries since I had all but given up hope fighting him after awhile. Now I can just breeze thru him. Nameless King took me longer and I was losing my mind but I eventually killed him and finished him off by rolling into with the Thorn Armor as an added bonus.
Have fun with some of these bosses as you truly attempt to git gud!