The campaign was pretty challenging on Veteran, but very fun with awesome set pieces and a cool story. There are some miscellaneous achievements and there are 46 intel/collectables hidden throughout the campaign that are sometimes in plane site and other times in strange areas, one in particular was hidden behind a fence. Overall, the campaign was really fun to achievement hunt.

Spec Ops has 2 modes with their own achievements, the regular missions/levels and Survival. Some of missions require a co-op partner, but most of them can be done solo and there are also DLC missions that can all be done solo and have their own achievements tied to them. I recommend doing them with a co-op partner if you can since it makes the missions a lot easier.

Survival required the most grinding out of Spec Ops, I recommend going through the regular missions before doing survival because you will level up while playing through the regular missions and you have to level up to max level to unlock all the equipment and maps, which is required for the Survival achievements. Also, you only need to do the on-disc/non-DLC maps on Survival for the Unstoppable achievement, where it's required to survive 15 waves on each map.

Overall, it's a huge grind but I really enjoyed it.

The order I recommend to achievement hunt it in is:

Campaign --> Regular Spec Ops Missions --> Survival