how to kill the most difficult bosses in genshin?

Do some jungle mining killing monster strong ones

buy characters, then you'll hit for 1 mil every time

you can clear with invite other people !

There is no such thing as a "difficult boss" in the game. If you have the right characters, and a well built team you could dish out a ton of dmg within a few seconds. I'd prefer to call the "difficult bosses" the annoying ones, which is time locked by spawning other stuff like the Oceanid for example.

I havent played Genshin in a while I might play some today!

After 6-7 world lvl if you solo player - its time to think about items what ur character wear and builds in this game. Its not only about feed them to higher level, but also grind for some special stats in item for better use of character. Yup its kinda boring. Or just find some high level teammates what will happy to help