So I was having a lot of trouble getting this achievement. I had tried the first guide and had some trouble following directions. I tried the After Party skull method, but ended up wasting too much time replenishing units and sending them back out. I think that both of these methods could potentially work, but I don't think my playstyle was adapting to them quite well. If you've tried both the above solution, and the After Party method, there is a third option.
Personally I found the TrueAchievements solution in the walkthrough of this game much more helpful. It's very similar to Segendary Ty's guide, but it's a little more elaborate and it approaches the level from a different angle. I will summarize what the section of the walkthrough says in regards to completing each objective, and afterwards I'll detail what I ended up doing differently.
Mission info:
Par time: 45:00
Gold medal threshold: 160,000+
Optional objectives:
1. Rescue India Company's captured Wolverines
2. Find and defend India Company's Nightingale
3. Kill both Goliaths
4. Activate all 3 Banished holo-projectors
Bonus Objectives:
1. Eliminate 8 Controller Sentinels
2. Lure Sentinels near 5 Banished buildings to destroy them
3. Use Holo-Decoy 10 times to distract Controller Sentinels
Do not use the Nightmare skull. Being able to save is a fundamental part of this strategy. The idea is to beat the mission in or at least close to par time. Par time contributes a massive amount of the points you will need, and if you're good at multitasking you can probably complete each objective in quick succession.
Optional objective 1: Rescue India Company's captured Wolverines - This is something that is not time-based, so if you want you can wait to do it last. I finished this before I went to the second Banished base (the one closer to the boss fight). By the time I got there I had already killed the three Controller Sentinels guarding the base. Just destroy the power core that's powering the shields and you'll free up the Wolverines. I would recommend reaching your population cop before doing this, as you can then get a bigger army, but also have less space taken up by some otherwise not-so-great units.
Optional objective 2: Find and defend India Company's Nightingale - This objective can start not long after you've gone through the teleporter according to a few different people, but for me it didn't start until I had claimed a second main base. The Nightingale can also be in a few different locations each time the objective spawns, so keep an eye on your map to help you identify where it's at. Save your game before you attempt to save the unit. Make sure you have Restoration Drones ready to go, and send some fast units over to counter the attack once you're there.
Optional objective 3: Kill both Goliaths - On this level, the Banished have two separate bases. One closer to the boss of the level near Point D, and another on the corner of the map between points E and B. Each bass has one Goliath guarding it. You'll want to take the base between E and B for yourself, so you'll naturally end up killing it there. The other Goliath will be at the other base next to Point D.
Optional objective 4: Activate all 3 Banished holo-projectors - The first one you must activate to progress in the mission before using the teleporter. The second one is just behind the base you will want to take, the corner between points E and B. The third console is located on the edge of the map between Point D and where the main boss spawns at the end of the level. I sent my Wolverines from the first optional objective over to this panel, as it was guarded by sentinels and pretty far off from where I was sending my army at the time.
Bonus objective 1: Eliminate 8 Controller Sentinels - Two of them you will kill as part of progressing through the level naturally. A third one spawns on the other side of the teleporter, and you'll want to leave him alone initially. The other five will be summoned to each point as you take it. Three of them will be guarding the Wolverines, while the other two will be at the back of the level where the boss spawns in. Obviously be sure to finish the other two bonus objectives first before completing this one. If you're having trouble killing these things and you're Holo-Decoy power is still recharging, select all units and use RT to cycle through different unit types. Command each one to go into a different location, forming a circle around the Sentinel you want to kill.
Bonus objective 2: Lure Sentinels near 5 Banished buildings to destroy them - This should be the first objective you focus on. Once you're through the teleporter lead your troops away from the Banished base. The Sentinel should destroy it on its own, immediately getting you 4 out of the 5 structures you need. You can then do one of two things; without destroying the Banished mini-base at B, capture Point B and then run away. The Sentinel should run into little to no units at all before destroying the mini-base on its own. If you mess that up, you can do something similar at Point A with the nearby Sensor Tower. When capturing your first point, use your Flamethrowers while having your other units hang back. When you take your first point, a cutscene plays and your units can still destroy stuff during this cutscene. It was long enough for the hero to destroy whatever was next to the point. Flamethrowers however, did minimal damage to the buildings.
Bonus objective 3: Use Holo-Decoy 10 times to distract Controller Sentinels - This is generally something you'll make progress on throughout the level. This is a very useful ability when fighting Controller Sentinels. I found the best time to finish up this objective, was after I had captured Point E. I just kept sending Holo-Decoys to the remaining sentinel until I finished the objective.
Some other things to note...
-Only have two Nightingales at a time. Everything else should be units from objectives, or Hornets. You're going to lose more units this way, but having the extra hornets ensures you're destroying things faster, helping you move quicker overall.
-If you're in a fight and you see your population decrease enough, call in ODST's and a Grizzly battalion to keep that battle going. The Banished will replenish their buildings and their units very quickly, making it very annoying to have to constantly remount the same attack over and over.
-Focus on upgrading your Hornets for more health, and damage. After that, research Reserves at an Armory to help you replace units quicker.
-Before you move units away from a main base, make sure that base has four turrets upgraded to Anti-Vehicle status. This should keep your base safe from the constant Banished attacks.
-Save after completing each objective. Try to keep saves from various times too, don't keep overwriting the same one. If you missed the gold medal by only a few points, and you didn't hit par time, you can likely just improve your time to get a better score.
-Don't bother trying to destroy the Banished base next to Point D. I wasted so much time attempting to do this. I found that they were able to build units much faster than I was, not to mention they have a seemingly unlimited supply of resources, so if your first attack doesn't work, there's pretty much nothing to do about it. By the time you come back with a second army, they're right back to where they started. You also just don't really need the resources from it. Instead, I sent my army around where the boss spawns in, and had them come in from behind the base. I had everything move ahead of Point D, basically to the back of the Banished Base, while I had my hero capture the point. Once the point was captured, I just left. Skipping destroying the Banished base will probably save you around five minutes or so, just be sure that while you're getting Point D that your Hornets find and kill the Goliath here.
-Right before you start the boss fight, set a global rally point just outside of the area. This way when you call in units to replace your destroyed ones, you don't have to manually call each one over as it spawns. You can set a global rally point by holding up on the D-pad.
-During the boss fight do your best to stay outside of the swarms that the Retriever Sentinel will call in. Save your Archer Missiles ability for the second or third time it calls in a swarm, when it's mobile. To get rid of the swarms you have to destroy the medium sized sentinels, but these sometimes move a little slower than the swarm itself, making it a little more difficult. Using the Archer Missiles on the sentinels can sometimes cause the swarm to dissipate not long after it arrives.
-Also during the boss fight, the Banished like to attack your bases one at a time. They destroyed my secondary base, and had just gotten to my first one as it ended. I'm not sure if losing all your bases causes a game over, but I would save before the boss fight and just ignore defending your base.
If you'd like a more detailed and chronological version of this strategy, check out the walkthrough for the game on TA. Partial credit goes to EmagDrolBot and Itzz Sh0wt1m3, the respective owner and overseer of the walkthrough I took most of this strategy from.