I did this on Easy, with the nightmare skull active.
You must also get all the objectives and gold on the same run through the mission.
1. In general, my focus was to get the resource containers in each area ASAP, as you need to get quite a few for the objective.
2. While getting the resources, if I could build on locations, I built a barracks and power extractor for upgrades, then Apex for engineers(after 5 Engineers, replace it with a Harvester)
3. Build on all defense locations possible(don't waste energy upgrading defenses), then use the Hero to meet the main flood offense, and the rest of your units as other defense. In the 2nd defense area, you can station most of your infantry in front of your base on the little 'cliff' like area, and the flood will run straight for them and get shredded by all the defenses and defenders!
4. The main units I focused on building were around 5 engineers, a lot of elite rangers, and then some hunters(upgraded if possible) in the last two areas. The hunters are especially powerful when garrisoned and the elite rangers are excellent for range combat.
4. In the 3rd area, don't forget to rescue the locust crew to the south! The locusts with hunters take down the Flood tentacles very quickly.
5. You'll have quite a bit of time in each area to set up defenses--try to do it quickly though as destroying the resource buildings and running your units around adds up time quickly.
6. In the final area, be ready to run your Hero and as many units as possible to rescue the 3 Locust crews ASAP!! There are two too the south, and one to the NE. There are several waves of flood that attack each Locust crew so be prepared.
7. While rescuing the Locust, make sure you are building up the base--you want at least 2 Harvesters, 1-2 Power Extractors, 1 Barracks, 1 Apex, and a War Council. Build FOUR shield Generators on the base turrets! Also, run some infantry to the 3 locations for Blisterback recovery. Build and upgrade those turrets with anti-infantry, and leave some infantry at each location for the rest of the mission. Also... capture the two power nodes after rescuing the locust.
8. Retreat to your base with the Locust, and build all defense locations and barriers, train hunters and garrison them in garrisons, station the locust just inside the base shields on all sides, place more hunters inside the base shields on all sides, put the engineers near the garrisoned hunters, and your Hero at the main enemy attack.
9. Continue upgrading units/war Council upgrades as you can, and make sure you have max population of hunters, rangers, and engineers!
10. Doing these general steps should easily get you all the objectives and a gold medal. Just make sure you watch your Hero's health and retreat him inside the shield if necessary.
Optional Objectives:
1. Recover 3 Blisterback Crews(markers will appear on the map in the final area, you do NOT have to keep them alive)
2. Rescue all 4 Locust Crews(One in 3rd area, 3 in last area, do not have to keep alive)
Bonus Objectives:
1. Recover 3000 resources(You will want to chase these down in the first 3 areas ASAP, and you should get this objective in the last area after a few resource structures have been taken down)
2. Build 20 defence structures(pretty much just build on every location, and you'll get it in the last area as the flood like to destroy your defense structures on the west side)
3. Kill 1200 flood infectors (don't stress if it's not happening, it should be achieved right near the end of the mission)