Can I fight lower level bosses in Genshin Impact?

Thanks for the contributors in the comments, I think I have discovered the issue.

Genshin Impact has a lot of things that you need to level up around your characters, rather than just the characters main experience.

1. Character EXP
2. Talent levels (Dependent on their ascension level)
3. Weapon level
4. Artefact levels
5. Character ascension level

Some of my characters were level 40, but had a full set of +0 artefacts on! I think the game assumes you have levelled all these pieces once you reach World Level 4.

Some advice I've researched on YouTube suggests picking one or two characters to focus on in the game, in order to push all these various levels up in sync with your character levels.