Halo 3: ODST Legendary Co - Op?

[joshedafox]

Me and my friends are playing campaign on Legendary. Half way through I had to leave so I left and then the other two guys continue. Then when I returned they had got to a checkpoint and we continued. I have just checked and it says I have not completed that level on legendary, and I need the achievement. So do we do the rest of the campaign then do that level again or do we have to do the whole campaign all over again?

 

[HeuerTag]

If you've finished all the levels except for that one on legendary (it should show you if you look at them what difficulty you've beaten them on) then you just have to replay the last level.