Reactor (Halo Wars level)/Walkthrough
From Halopedia, the Halo wiki
Start to build your base with Turrets, two Reactors of which one should be advanced, a Barracks (although once you get the ODST upgrade, you can destroy the Barracks to clear up space for other buildings), a bunch of Supply Pads and a Field Armory to get the Population upgrade as well as the Adrenaline upgrades, after which you can also destroy the Field Armory.
With the base up and running, send several units up to the Covenant Base to the northwest and take it over. There will be several other bases that you need to take over, and every time you've made a new base it will be safer to transport the core over to the next area, until you've finally reached the Apex Base site.
Spawn an Elephant and connect it to the core when you've destroyed the base. It's basically a matter of repeating the first step; Destroy the Covenant Bases and move in with the core until you reach the Apex Base site.
If you want to collect the 19th Black Box, take a Hornet over the left side of the first cliffs that are just up ahead from the first base you start at. For the Skull in this level, destroy 20 Vampires and look just north of the 2nd (former) Covenant base to find it nearby crates.
When your Elephant (and core) reaches the Apex Base site, the mission is over.