In my experience, you do NOT want to have the nightmare skull on for this mission!
There's nothing worse than playing for 25-30 minutes, then failing because you can't save and the Flood super-unit breaches the perimeter! I know, it happened to me...
You also must get all the objectives and gold in the same run.
Put the mission on Easy. I was able to easily get the Gold score and all objectives on Easy without any skulls on.
1. Buildings on main base: 4 Harvesters(upgrade), Apex, War Council, Mega-Turret, Shield Generator.
1. Banshee production, with about 1 Engineer for every 5 Banshees, and Apex upgrades!
2. War Council upgrades
3. Turrets around the center of the map, a mega-turret on the main base, and Sensor turrets on all main and extra main base build locations.
3. Send Banshee fleets of at least 10 around the map destroying everything in sight, but particularly the yellow crystals.
4. Send a few Choppers with the Banshees to collect the yellow crystals. Use Choppers because the Hero is too slow to go with the Banshees.
5. Get rid of starting troops that are taking up roster cap space. Have them harvest crystals initially until your Banshee fleet is up and running!
6. Use your Banshee fleets to take out the two other main bases at the bottom left and top right of the map. Build 4 sensor towers on each of those bases to give yourself massive map Line of Sight, and also build some Harvesters, Apex/Barracks/Foundry in case you have extra energy, as it's useful to upgrade your infantry(your Hero) and your vehicles(Choppers).
7. As the mission goes on, you should be able to either use super-powers(The Lich is fantastic for stopping the Flood waves), your Hero, and some Banshees against the periodic Flood waves.
You do not have to activate the drills one at a time, you can wait until you have enough energy for them all.
When the Flood waves start coming, try to upgrade the turrets for anti-infantry everywhere with an anti-air here and there.
After activating all 3 drills, station groups of Banshees around the various entry points, and you should be golden.
Watch for the Flood Superunits! If you see one approaching(your extra bases with the max range Sensor Tower line of sight will spot them very soon), save the game! There will always be two at the same time, and they come from the exact same places, one on the left and one on the right. The one on the right side isn't visible until close, so be very careful with that one.
You should only have to take down 4 of the Flood Super-units on Easy. Be aggressive on them with your Banshees, charge in to attack them soon so they can't get close!
Remember, if the Super-units aura touches the drill, the optional objective is failed!
Before the end of the mission, make sure to fly your Banshees to all 3 Flood spawners, and destroy all 4 Flood bases to take care of two optional objectives.
Optional and Bonus Objectives:
1. Discover all 3 Flood Bursters
2. Destroy all 4 Flood Bases(one in each corner quadrant, NE,SE,SW,NW)
3. Prevent Flood Super units from reaching the drill(this includes their aura touching the drill!)
4. Destroy 10 Flood tentacles(super easy, your Banshees will wreck them)
5. Destroy 30 Flood with the Mega-Turret(if you have the extra energy, upgrade the Mega-Turret for extra potency, it's a lot of fun to watch after the nightmares in Halo Wars 1 Arcadia City and Arcadia Outskirts!!!)