My save is at 250h, which doesn't include demo progress since I started new.
I'd say it's easier than getting the slayer collection done. Instead of wasting time breaking parts you just go in with a strong weapon and aim for the fastest kill you can, since more kills = more shots at getting mitamas. I was killing Jollux for Susano'o in under 4min and farming Viper Queen in that 12min mission, meanwhile I'll spend over 10min breaking Chthonian parts and hoping for the right drops and that it doesn't die before I'm done.
There are some that can be annoying to get, though, like the few that can only be found in Four Horseman, Oni Onslaught or other multi-bosses missions that'll force you to kill bosses you don't need, and those from invisible oni that'll make you enter & quit the same mission until it spawns. But most of them are fairly straightforward to get.