Hello there.
Now I know this sounds a bit dumb. But you know, after playing a few hours of campaign and then going offline for the rest of the day, then coming back on and what not, I occasionally forget what mission I was on with regards to completing it on Hard/Versus mode for the achievement. Because I've already done all the campaign missions, the little circle next to each mission to show completion is already white, so I can't tell.
Another problem is that Im doing it in Versus mode and after completing a level, does it still keep you in Versus mode when moving onto the next map without going back to the main menu and searching again for the next level. Because as I complete a mission in versus, it always moves onto the next level, but I seem to have done several missions now in versus mode against only bots. Thus making me believe that if you dont go back to the main menu after a mission and search for a match for the next mission and physically choosing versus yourself, it may not be versus in the next mission you play.
I know that was quite convoluted, I find it hard to explain but I hope you catch my drift lol.

tl;dr Maybe a campaign mission system like Halo, in terms of when you complete a mission, next time you scroll onto it in the mission list, show what difficulty you have completed it on, or whether it was solo, versus or co-op.