Field Boss Sketching is EASIEST, hands down.
If you have a little common sense, you can easily figure out what to do.
Get a couple friends, and have everyone idle (or explore, or grind or something) in Rano on DIFFERENT channels.
If you can get you, plus 5 other people, you can occupy every channel, and while everyone is busy doing stuff in Rano, if a boss spawns, a person will notice the text, and can easily message all the friends they are helping.
After getting the sketch, head back to your respective channel, and wait some more.
Me and one friend did this ourselves, having occupied two channels, we found field bosses plenty faster then we did when we waited in one channel.
Took about 2~3 days, but we only did it for a few good hours a day, so at most, it could take 24 hours-ish, total.
If you occupy every channel with people who will tell you when a boss spawns, you can probably get all bosses sketched in a fair daily-playtime.
Much more easy then running a dungeon that you can't even complete on your Elf/Giant, 7+ times.