Halo 3 Co-op Campaign on Normal?

[peppermagik]

My friend and I played Halo 3 on co-op on Normal and beat all the missions, when we finished the last level Halo we did not get any achievement for beating the level or the game, why?
I had not beaten it before, neither had he.

 

[Lasagna delivery guy]

check the very first mission "arrivel",which is just a cut scene but also counts as a mission.watch the whole cutscene and you finished the mission

 

[Charlie]

Just play halo on Legendary and you will unlock the Achievemants