Beating Halo 3 on legendary...

Ok, when I go to mission select, there is a SOLO and COOP space for a shield after you beat it on a certin difficulty..So my question is:

Do you need to either beat it SOLO or COOP? Or can you do a mix, just as long as you beat all the levels on legendary?

the game is a cakewalk, doing legendary on co-op is weak.Weapons_Free

If thats your opinion. I went through on solo and beat it, then let my brother play and I stayed as his "respawn point" so he could beat it. Actually a good strategy if you are lacking in skill like my brother.