I think it depends on the boss. There are some I can solo fine, or just use summon spirits.
There are some where it really just helps for you to not be the only target. And it also depends on builds. I typically run casters, so I can't get right up on the boss and really benefit from having someone tank the boss so I can do my thing.
We can disagree on a melee build being the best way to engage most with the content
I'm glass canon or bust.
Also I will use my OP weapons all I want. You can bring that melee build around my Moonveil and see what's up.