Overall, I found "Awakening the Nightmare" much more difficult than the base game on Legendary, due partly to a greater variety of mission objectives. Still, there's nothing that left me tearing my hair out like missions past (*cough cough* Halo Wars 1 Legendary) so if you're familiar with the Banished faction and decent at the game it shouldn't require multiple tries. Some general tips:
+Turn on positive buff skulls. It'll kill your score, but playing methodical and safe is a better strategy than going for gold anyhow, unless you're a masochist. I'm not judging.
+Learn to use Engineers. These guys will keep your defenses and siege units alive, and are invaluable; however, keep in mind that your population is not infinite and a fighting force that's mostly engineers versus damage-dealing units is not a good fighting force. Aside from air infectors, who can dive-bomb and kill them, the Flood will almost never attack the Engineers, allowing a few of them to hold positions that would otherwise be overrun by keeping that turret or siege unit alive. They're great!
+Reduce your use of infantry. While Jump Brutes and Grenadiers are fairly effective against the Flood, as a rule vehicles and aircraft are less prone to infection and the smarter choice.
Mission 1: What Could Go Wrong?
+ Early on rotate out units being aggro'd by the Sentinels. You especially want to keep the Locust alive since you can't build them this mission.
+ Don't try to capture and hold every minibase. You won't have enough EMP charges to stop Sentinels from pouring in. On my legendary run I ended up constantly blocking the NW and N portals; my death ball of units kept itself healed up no problem so I never had to worry about reinforcements.
+ Upgraded turrets and upgraded fortifications are beastly in this mission and campaign in general, so don't neglect them.
+ I suggest being proactive and helping to knock down barriers/scouting ahead and dealing with threats before they reach your Scarab, rather than as they come.
Mission 2: Fighting Retreat
+ This is probably the toughest. Make sure you keep an eye on your goliaths and rotate some engineers and Pavum to keep them alive.
+ When you reach the final hold point, quickly get the Blisterbacks online. They will help greatly, and if you leave each point with a turret and an Engineer or two, they should be able to stay alive until the very end.
+ I ended up grabbing one energy node to the east and the reinforcements/resources there, but chances are you're going to be pressured pretty heavily throughout the game, so rescuing the other units is probably not a wise use of your time compared to making sure your base stays safe.
+ If you find yourself getting steamrolled by the final minute, don't worry. Grab your forces and cram them against the edge of the map leading to the final zone. I thought I was a goner when the Flood overwhelmed my defenses with a minute left on the countdown (plus an interminably long time of back-and-forth dialogue before it unlocks the rest of the map) but the Flood focus on taking down your base and will mostly ignore your units so you can sprint the final way.
* Spam your leader powers to clear the route when you're fleeing!
Mission 3: Light the Fuse
+ The trick here is to balance upgrading your units with not having to spend forever scouting more energy crystals to get the drills running. I ended up taking one additional base to the east, but given that you can't produce any energy structures additional ones probably aren't worth it. Intelligently splitting up your forces to deal with multiple avenues of attacks is the secret to keeping things together. Mega Turret + Blisterbacks + Engineers do a lot of work here.
+ Don't stay out in the field too long when the pressure builds up. Depending on your unit composition your expeditionary forces will be slow, and if you're only moving back to base when it's at 95% or so you could have your defenses severely damaged by not being in position. Against the Flood, overwhelming damage is the best play, so always make sure to intelligently concentrate your forces.
+ Upgrade your mega turret! On this mission I went with lingering damage to help damage the clustered units.
+ When the Abominations come a-knocking, use stasis on one side to slow them down while you focus fire on the other one. The Lich and glassing beam also do wonders on relieving pressure from a lane while you deal with other threats first.
+ Remember your engineers can repair the drill, and pay attention to their health! I realized almost too late that infected Banshees were chipping away at them and my turrets weren't killing them fast enough.
Mission 4: The Archive
+ Tank damage with your Hero unit. Incendiary gel wrecks infection forms, so use it liberally and don't be afraid to wait for it to come off cool down before proceeding.
+ The key to this mission is not fighting units you don't have to. The supply caches off to the sides and up on cliffs often have Flood tentacles guarding them, but rather than try and kill them, just destroy the resource containers and scoop up the goodies while microing your units out of the telegraphed impact zone.
+ When you reach the malfunctioning Light Bridge, I sent all possible units over to the safe point up north and had Voridus tank the entire wave safely with infusion and Cataclysm.
+ Use Voridus to micro through the energy beams. He can take a few hits if you mess up, no problem.
+ The end might seem like another impossible task, but remember: all you have to do is basically keep the countdown going. So if you run into a mass of forces you can't kill, just kite them, using Infusion and reinforcements to circle the perimeter, clearing clocks when able and focusing down bosses where possible. This will probably take some time, but by the end I had max population and was keeping two locks cleaned constantly while the big enemies were on the other side of the dias from me.
Mission 5: Manifestation
+ This one can feel like a slog. I recommend a unit composition of air units just because they can cover the terrain much faster.
+ Blisterbacks + Engineers at the turret sites can keep them safe with minimal attention from you.
+ You don't actually have to proceed with the mission: after you've constructed the first mega turret, if you hang back and don't encounter the Proto-Gravemind, basically nothing attacks you. So take the opportunity to gather a sizable force and max your upgrades. That way you can quickly steamroll your way from one base to another.
+ Your home base will get heavily attacked throughout. I recommend leaving a couple of engineers back home to repair the turrets and combining that with the Lich leader ability. That way you can mostly leave it to defend itself.
It'll take some time, but the campaign is manageable. Just don't panic, and enjoy the sound of the popping achievement at the end!