Halo Wars: Definitive Edition

This is a fairly easy mission, even on Legendary. The AI will keep sending Banshees at you though on Legendary, and they will wreck your Turrets early on. This mission was easier on the 360 because you could build Cyclops to repair your turrets and buildings.

My strategy for Mission 13: Beachhead

The overall focus in order of importance:
1. Take out the Covenant base that's just south from Anders departure position(after Anders has left the Battlefield and the Spartan's have arrived)
2. Build up the base very fast.
3. Capture the Scarab and use it to take out the right side base and the center.
4. Get the ODST upgrade and all other useful upgrades.
5. Assault the last Covenant base.

My general walkthrough steps:
1. Take out the Covenant base that's just south from Anders departure position(after Anders has left the Battlefield and the Spartan's have arrived)
2. Have your Spartan's hijack as many Covenant vehicles at that Covenant base and take it out ASAP.
3. Send your healthiest Marine group to the Supply Tower just North of your base.
4. Have all your other troops guard your base by standing near the left front corner.
5. Get 4 Turrets with Anti-Air on them up.
6. Build an Airbase, and at least 1 hornet.
7. Send the Hornet as soon as you can over to the right into the field there--it'll discover a Scarab the Covenant have abandoned, and will start shooting the Flood holding the Scarab.
8. Once the Hornet finishes, or before, you need to transport a Spartan over there and he'll help with the flood. Use the Chain-gun Spartan if you can!
9. Hijack the Scarab with the Spartan, and send it up the little road up the hill to the right first--that will eliminate one of the Banshee spawn points!
10. Then take the Scarab towards the Covenant base on the right side, at the bottom of the cliff and park it where it can slowly work through the bases shields.
11. There will be Covenant reinforcements that drop in at around half base health. Back away if necessary from the Hunters, and take them out with Hornets/Scarab.
12. Use your Heal Leader Power generously on the Scarab! You DO NOT want to lose it.
13. Once the right side base is captured, and fully turreted, consider parking an army of Gauss Warthogs or Hornets just to the North of it to defend against Banshees.
14. Use the Scarab with a few Hornets and/or ODST's to take out the center base. Make sure to use the ODST's Rocket ability as often as possible.
15. Take out the center base, quickly build it up with all Anti-Air turrets, and prepare for the final assault.
16. There's two ways you can do this--I always used the teleporter above my 2nd base, however I think you can just go all air units and take out the final Covenant base from the left side avoiding fighting the other Scarab! If someone tries that, please let me know. 2-3 Vultures with the Mega Barrage upgrade and some support Hornets and a lot of ODST's should make quick work of that base from the left side.
17. Either way, take out that base--if you use the teleporter on the right side, kill the Scarab using everything--it will quickly upgrade it's stars killing your ODST's, but your Scarab and Bombing Run should take it out fairly quickly as well as the ODST's rockets.
18. Build on the final base location and Victory!

The Banshees spawn from at least 3 locations. There is one spawn on the hill on the right side. Another spawn point just to the left of the final Covenant base. And I believe the third spawn location is to the right of the Covenant Scarab you don't get to hijack. It could be worth sabotaging those Banshee spawn points early on with a Hornet/Bombing Run attack just to save your bases from the constant and annoying attacks! I never tried this, if you do please let me know how it goes!