Every boss is hard for different people, for example
If you have 3 cryos and 1 pyro, and need to fight the cryo regisivine, it would be hard, since you can only use ur only pyro character, really.
However, If you don’t have any cryo in your team, it’s gonna be easy.
But there are bosses that are harder than the rest:
The weekly bosses. Such as;
So I would say there’s really no “strongest” boss in genshin, your team and playstyle matters alot.