Halo Wars: Definitive Edition
75 Achievements
35-40h
Drain Cleaner
Kill 20 enemy squads with a continuous Covenant Leader Power on Memorial Basin.
1.1%
How to unlock the Drain Cleaner achievement in Halo Wars: Definitive Edition - Definitive Guide
2 v 2 Deathmatch on Memorial Basin, choose Heroic for the difficulty. I chose Arbiter's as all 4 leaders (important not to be against UNSC leaders as they can disrupt your leader power quite easily).
Arbiter is more agile and easier to keep alive than the Chieftan and Prophet imo. As well as the advantage of being able to walk around the map freely in your leader power.
When the game starts, build your leader temple and simply fill the remaining base slots with Warehouses and Shields. You can occupy the free base as well to build more Warehouses (although the 15k resources you start off with is easily enough).
Activate Arbiter's leader power and head straight for the enemy base(s), they should have multiple units out already. Start smashing the daylight out of all the enemy units with the Right Stick, you should amass 20 kills in no time.
Just make sure that you are always in your leader power, you will know this as the camera will be zoomed in really close on the Arbiter. Don't ever press Y after activating it.
Took me 4 minutes from start to finish.
Update: Easy with a controller. Not possible with m/kb, because of how the controls get you stuck.
Play 1v1 on Easy as the Arbiter with Cutter as your enemy. Build a Temple and Warehouses. Move the Arbiter towards the enemy base and wait until they have a small army deployed. Enter the Arbiter’s rage mode and keep moving from unit to unit. Ideally, Cutter will continue to deploy units and you will hit 20 during this time; however, sometimes he focuses on building up his base so you can either wait until he fields another army or restart.
It can be a little tricky as there is an element of luck involved but as long as you have enough supplies coming in to maintain rage mode, you should be able to take out 20 units in one go. I found playing as the Arbiter to be slightly easier than the Brute Chieftain or the Prophet of Regret.