Legendary walkthrough for this mission!
I played this first time and FAILED. After 20 minutes. You WILL fail too first time unless you know a few things!
First of all, the Banished will keep coming in stronger and stronger waves through this mission. If you follow the missions 'tips', you will most likely fail.
And, there's a huge 'exploit' that's totally worth using to make this a lot easier.
Steps to beat this on Legendary:
1. When the mission starts, you can pretty much ignore the starting attack, although do send your Spartan in to get downed on the beach.
2. You can't really stop the attack there, and you DO NOT want to stop the attack there!
3. After losing all your troops on the beach, the Banished will just wait there, and if you don't kill all the Banished units, the wave timers won't start!
4. So, leave some Banished on the beach, and proceed to max your base/troops/garrisons/towers everywhere else.
5. Build up your base, go for 4 Supply and 2 Energy, get them upgraded as soon as possible.
6. Add an armory, then an airpad/foundry on the mini-base to the NE and a foundry/barracks on the N mini-base.
7. UPGRADE EVERYTHING! Especially the Armory upgrades! UpgradeUpgradeUpgrade.
8. Then focus on building units. You want 5 Wolverines near the NE beach(about between the garrison and the tower) with 3 Scorpions and 2 Nightingales. Also 2 Flamethrowers in the garrison there with the Tower upgraded to anti-air defense.
9. You want 1 Nightingale near the SE garrison(a little behind it) with 1 Marines and 1 Flamethrower in the garrison.
10. You'll want 4-5 Nightingale's at the main beach, spread between the garrisons and your units. 2-4 Scorpions. 2 Flamethrowers in each garrison. And the Spartan in the middle of the beach. You may also want to try some Cyclops, but the Locust kill them SO fast it's ridiculous.. they can be useful though if the Spartan takes the fire first. Oh, and upgrade those two towers with anti-vehicle upgrades(only do this when you're done upgrading everything as they might take out the Banished left on the beach!).
11. That's your basic setup. Now, you basically just have to micro-manage your Spartan for the attacks on the main beach, and be ready to build replacement units fast. The Banished likes to target your Nightingales at the NE beach with the Leader Power Glassing Beam, so be ready to spam Nightingales and Flamethrowers.. the flamethrowers in the N beach garrisons die a lot, but are critical to replace fast to take the fire from Banished troops allowing your Scorpions and other units free fire on them!
12. Once in a while you'll need to send a scorpion down to the SE garrison to help out.
13. When the locust show up, try to always hijack one with your Spartan, and take the fire while you attack the others with your Cyclops/Scorpions.
14. You could optionally try spamming a LOT of Hornets, but that will probably only be effective on the North beach, and not very.
15. A Scarab will show up after the "Defend your Base" objective has been completed. It was actually a piece of cake when I called ALL my units in to attack it--it retreated once into the water, so I backed away, put some more Flamethrowers in the garrison there(it focused on the garrison and I was able to unload on it LOL), then hit it again and it'll be over soon with your Scorpions and Nightingale's doing their things.
I hope this helps! This mission was a pain at first, until I realized that upgrade trick!