Question about Legendary Campaign

Games Other Halo
1

Ok so I’ve read that in order to unlock the Mk VI Helmet I need to complete the campaign on Legendary. I just have 2 quick questions about that.

  1. Does it have to be solo or can I bring some friends along? If it can be done via Co-Op, I’d definitely be up to tackling this sometime soon.

  2. How hard would you say this campaign is on Legendary? And how is it solo compared to Co-Op playthroughs?

Thanks in advance for the help guys.