On the contrary, Halo 4 is too easy to be fun on any difficulty below Legendary. Look, your network mode has to be set to Xbox Live, so go to your Spartan Hub each time you start playing.
Also, you can't go by rally point. If you're co-oping, each mission must be done in one go. You can do multiple missions in one go, but if you, for example, do one mission and half of the next, then quit on co-op, you have to restart the next one. No way around it.
To be on the safe side, go solo Legendary with no skulls while your network mode is Xbox Live and don't replay any levels in between. No going up to Level 5 then going back to play Level 3 on Heroic, for example. No adding skulls or changing your rally point. Just play until you save and quit, then next time you play campaign, change nothing at all and just resume from where you left off.
Also, to know for sure, each time you complete a level, you will see a commendation bar on the right of your screen pop up. If it doesn't pop up, save and quit to check your commendations, and replay the level if it doesn't show up.
I just did a straight Legendary playthrough in a single weekend, following the advice I just gave you strictly. Got Mark VI no problem.
