Originally posted by GunsForBucks:

Go into console mode and
listkeys
This should show you all the world level triggers

Copy the ones you want to keep

resetkeys
This will remove all the progress from the world level

Add the ones you want that you copied

source
https://www.reddit.com/r/valheim/comments/ly8e1j/resetting_boss_flags/

I didn't know this. I'm going to try it right now. Thank you very much friend.