Halo Wars: Definitive Edition

Edit: Dr3ads Wrath and a rabid fuzzle confirmed that you need to keep the ODST alive as well. Keep an eye on them and use your regen ability if their health is low.

Short version:
Play on easy. If you have skulls that help you out, put them on. Clear the objective and put a fully upgraded scorpion and a fully upgraded wolverine at this location. The wolverine will kill all air units and the scorpion + dropped ODST will kill any ground units that were lucky enough to land (once a Rhino is placed Cutter will send some ODST reinforcements). Now send your Rhino and rinse and repeat for the next Rhino location.

Extended version:
1. Grab the 2 additional reactors you have next to your base with your marines.
2. Build 5x Heavy Supply Pad, Reactor, Armory then Research +10 units. Also build 4x turrets with anti-air upgrades.
3. Destroy Armory and build a vehicle depot.
4. Upgrade your reactor to get Tech level 4.
5. Upgrade your Scorpion with canister shells. Upgrade your wolverine to dual launchers.
4. Now queue 9 scorpions and let your Spartans occupy 3 of them. Also queue 5 wolverines.
5. Send a scorpion and a wolverine to position 1. Send the other scorpions to position 2 and leave the wolverines at your base to defend the big square in front of it.
6. Order Rhino #1 and send it to position 1. A base will spawn at position 2, use the scorpions here with canister attack to destroy it immediately.
7. Leave a scorpion and a wolverine at position 2. Gather the other units at your base and order Rhino #2 and sent it to position 2.
8. Leave a scorpion/wolverine at position 1 and a scorpion/wolverine at position 2. Send 3 Scorpions and a wolverine to position 4 and 3 scorpions and a wolverine to position 5. Make sure both locations have a Spartan-tank. Order Rhino #3 and send a transport pelican with this Rhino and the remaining scorpion with the third Spartan and a wolverine to position 3. Be careful not to position your Rhino just yet!
9. Once cleared position 3 will not be attacked again. You can safely send the Spartan Scorpion back to your base. This scorpion is handy to guide/guard the next two rhino's to their objective. If for any reason this position3 will be attacked the wolverine will take care of it.
10. Safe game! Check the health status of Rhino #1 and #2, if necessary use your heal ability. Now place Rhino #3
11. Immediately order Rhino #4.
12. In the meantime clear the new base at position 4 with the scorpions present, use canister attack! Do the same at position 5. Use healing if necessary and send a new scorpion if one gets destroyed. This is the hardest battle you have to fight, because covenant reinforcements will remain attacking these areas.
13. Send the scorpions at position 4 and 5 a bit downward to check for any covenant forces. Sometimes enemies are lurking in the corner area you have to pass to position 5. Sometimes even a small structure will spawn. Use your scorpions in this area to clear the path from the base to locations 4 and 5.
13. Once safe, send Rhino #4 with escort to position 4.
14. Safe game and Order Rhino #5.
15. Check if any Rhino's are low on health, if so use your heal ability. Also check the status of the protecting units, if any units got destroyed replace them.
15. Check the route to position 5, if safe use the Spartan-Scorpion to guide the last Rhino to its objective.

My strategy was to do this on easy difficulty, and making a large army of Gauss warthogs and keeping 5-6 at each rhino when they are at their positions. Better yet, you can keep some more on the bridge to the second rhino location and at the first rhino location to kill any spirits before they drop infantry, and any banshee that flies over it. Putting the warthogs on bases 2, 4, and 5 before they unlock will also make quick work of killing the enemies units and buildings right as they spawn.