Halo Wars: Definitive Edition

This achievement takes a lot of patience and an equal amount of preparing. Since you cannot claim any other bases for this mission, you're stuck with one base: that's only 7 slots total. For units, you have Forge in his Gauss Warthog, 3 Spartans, and two Wolverines, plus the Reactor you need an Elephant to haul up to the goal.

I did this on Normal, but can be done on any difficulty. There are 3 Covenant Bases to contend with plus any units they throw at you, one small area loaded with Flood enemies, and the occasional Sentinel fliers. You could try to rush your way up through the map, but on higher difficulties that would be suicidal since the Elephant towing the reactor isn't build for speed.

To start with, upgrade your Base to Fortress, giving you your 7 slots to make the following: 2 Reactors, 3 Supply Pads, 1 Barracks, and 1 Field Armory. Make two Turrets when you can to boost the Base's defenses as occasionally airborne enemies will attack your Base.

Use the Spartans and Forge's Warthog to circle around the perimeter of the base to found up to 3 piles of extra supplies, and then have them scout ahead up the first incline to take on any small fry enemies. Then have them go for the first Covenant base, taking out the front two Turrets and any front buildings. If you're lucky, the Covenant haven't finished filling out the base and you can easily destroy one building being created.

While you're attacking the enemy base, upgrade your Supply Pads to Heavy Supply Pads, and create an Elephant to tow the Reactor (the objective of this mission) and an extra Warthog. Upgrade the Warthog to give it a Gunner and an grenadier, and upgrade the Elephant to give it Twin Engines. At the same time, upgrade Marines to give them New Blood and Rocket Launchers but do NOT make any marines yet! Research the following at the Field Armory: Reserves and Adrenaline (and Medium Turrets to boost your base's defenses). Build one or two more Turrets for your base if you choose to.

Send all your units to finish off the Covenant base and let them stay there for a bit to keep the Covenant from reclaiming that spot and to stave off the Covenant and Flood enemies that will come down to that location to attack. Remember to NOT build a base here, otherwise you'll have to restart the mission over again. Use Regeneration to heal where necessary, but don't send out any units yet up the map.

When you have enough resources, upgrade one of your reactors so you can research the following once you are able to: Medic (Marines), ODST (Marines), Reinforcements, Gauss Rifle (Warthog), and Defense Turrets (Elephant). If you want, you can max out the upgrades for the Flamethrowers as well - I did, but I didn't use any Flamethrowers in my run for this achievement. Once you've maxed out your Marines and have researched Reinforcements, destroy both the Field Armory AND the Barracks to replace them with an Air Pad and another Supply Pad/Heavy Supply Pad. Begin researching Wingmen for the Hornets.

Upgrade your second Reactor to max out your tech level and proceed to research these last few necessities: Chaff Pods (Hornets), Mega Barrage (Vulture), and Ceramic Armor (Elephant). Once these are researched, make 3 Vultures and at least 4 Hornets (With these, your should have 34 out of 40 units on the field). Send them up to the next tier to begin thinning out the Flood positioned there (especially the one that launches airborne projectiles at your units). Begin moving the rest of your units onward, sending your fliers further up once the Flood's been wiped out to work on the 2nd Covenant base and its defenses. Once the base's Turrets are taken out, bring everybody up to wipe out the base.

There's one last Covenant Base to deal with at this point, but given you've thinned out the enemies by now you can just dash for the goal point with the Elephant and have your units shield it from the enemies, or you can send everything you have at the final base and drop in ODST teams to help speed the Base's destruction before sending the Elephant on to its destination.

Again, this method takes time and prepping. Also, I went with fliers instead of land-based vehicles like Scorpions and more Wolverines as Hornets and Vultures moved faster and took less beatings from ground enemies. Once the Elephant gets to the goal point, provided you remembered to only have your main base and no others on the map, the achievement will pop.

Good hunting!

Pretty straight forward, do not claim an extra base when going up the hill. Doing this on easy or normal with all flying units will make this a bit more tolerable, so you wont have to travel the zig zag back to the top if you lose units.