*Edit* It has been confirmed that this can be completed in co op. I would highly recommend running through the campaign on easy/normal and getting all the skulls unlocked before trying this. Once I had all 15 skulls, I ended up turning on all the skulls that made it easier and let me tell you, with the skulls on you don't even need a guide. It's a walk through the park and I never had to restart once.
Skulls to activatePain Train - All player units train 50% faster
Bountiful Harvest - Supplies from supply buidings arrive 25% faster
Emperor - All player powers recharge in half of the time.
Total Annihalation - Spartan special ability slams create and even more powerful explosion upon landing.
Firestarter - Grunts and Marines explode on death inflicting damage on enemy units.
Things That Go Boom - Here units unleash a nuclear explosion on death.
Sugar Cookies - All player units have 50% more hit points.
Shadow - Player core infantry units start cloaked.
**Note** The only skull that is beneficial that I didn't turn on is After Party. It spawns units anytime a firebase is destroyed (never happened) and I just didn't want it messing with my population cap. However, it's been brought to my attention that it could be a strategy in itself to use that skull on the earlier missions, but it comes at a cost. Destroying an enemy base means they get units too. I didn't find it necessary but completely up to you.
Basic StrategyEverything was essentially getting air units and upgrading them to max damage/health. Obviously some missions won't allow that, but with the skulls on you shouldn't have any problems with any combination you choose. When aircraft was not permitted I upgraded marines and just kept pumping them out to max cap and assaulting. It depends on some maps, but have at least two anti-air turrets as defense on your base for the banshees are the primary pests that keep coming periodically just to make sure you're aware that your base should have turrets. Too easy.
Important UpdateFrom
Xavier0080:
Ok, so this mission can glitch (as of May 6th). Whenever you complete any mission when going for any campaign achievement, NEVER HIT "Exit to Menu." While the game will record your progress, the achievement counter may not. Always continue to the next mission and then if you want to stop hit resign after the auto-save.
I had to replay the campaign on Legendary until i found the mission that glitched for me (which showed i had already beaten it on Legendary), once I completed it the achievement popped. This same glitch can also happen to Skulls/Gold Medals/Bonus Objectives. Just play it safe, and never hit "Exit to Menu"
If you're still struggling, check out the thread of comments below. Some guys tweaked it to perfection for some of the hard missions like using pestilence to your advantage and turning off emperor because that also works for the AI as well.