Halo Wars

In the tutorial use your leader (the prophet) to kill all the units. Achievement unlocked!

As simple as that.

Actually, it is best for the brute chieftain. On blood gulch, simply build the temple and right away go to their base. If they have no units back off, but they should have a ghost nearby, and when they do, use his "y" attack and kill them.

IMPORTANT!!: Even if they continue to build units, all you need to do is kill all their units on the map at one given time, not throughout the whole level.

It will unlock after their last unit is killed by the brute chieftain at any given time.

Instead of these ways you can do it quite simpy online. Go into a 1 v 1 matchmade game. Be seargent forge for this as it is much easier. Send your beggining warthog over to the other base before you do anything and find his warthog. Whilst doing this make a reactor. He would most likely either be scouting you or collecting resources. Use your carpet bomb on him for the achievement to pop. Easy.

If you want to kill two birds with one stone, I got this achievement by accident while earning the Micro Manager achievement on mission 7 (Do not destroy any of the power nodes). On easy difficulty, build lots of supply pads while blasting away with your MAC gun from a distance. The catch skull helps the MAC charge faster. I'm not sure why this works since there are more enemy units on the map, but it does. Easy as that!

edit: The wording of the achievement made me think you need to destroy the
very last enemy unit on the map. The actual requirement is to use one of
your leader powers to destroy the last enemy unit needed for victory.
Since destroying the scarab equals victory on this mission, killing it with
the MAC while not touching anything else unlocks the achievement.

To build on S3V3N's method, on mission 7, load it up on Easy. Once the mission starts, move all your units back to the back of your base so as to avoid the Scarab's attention. Fill 3 of the 4 open spaces on the Firebase with fully upgraded Heavy Supply Pads and the 4th with a Field Armory.

Upgrade your turrets with Flame Mortars and Rail Guns (more of the latter will usually fare better), and use the Armory to upgrade them to Large Turrets which ensures you won't need to use a Heal and Repair.

As the above are building, begin unloading with the MAC Cannon.

If you have the Emperor Skull from the final mission, activate it so your MAC Cannon regenerates faster.

You don't ever need to send a unit out and by doing this method, you can also get another achievement for the mission, Micro Manager:

I would strongly recommend this on Chasm. Put it Forge vs. Forge on Easy. Build a Reactor and collect the resources right by the front of your base till you have 600 resources. Drive to the other base, his Warthog should be stationary. Bomb it. Achievement Unlocked.

- On Mission 7 build up your base and use the bountiful harvest skull and the pain train skull.

- when you upgrade to medic for your marines (Quickly i should add as the scarab might not be so kind after a while) send some units directly north (south of the scarab if you want to look at it that way) you will find a sniper tower.

- put your marines in there.

-the scarab should start shooting at your marine but oh would you look at that they dont die (the scarab shoots at the base of the sniper tower not at the part levitating. your marines should be fine. the medic is there just in case)

- now the scarab should be preoccupied trying (and failing) to kill your invincible marines and will not move around (move its head)

- now run around the map decimating everything (upgrade everything else as you see fit)

- when all but the scarab are obliterated continueously use the MAC cannon (upgraded if possible) or sgt. forges "Y" attack (I suggest only using the MAC) to destroy the scarab and you will get the achevemant (or at least you should if not then it cant be helped, try it again)