Drain Cleaner

Drain Cleaner is an achievement in Halo Wars tied to the unreleased map Memorial Basin. To earn it, a player must kill twenty enemy squads with a continuous Covenant Leader Power. One tactic to get this achievment is to use the Arbiter's rage mode against an army of infintry since the Arbiter in rage mode can be difficult to take out.

Walkthrough
One solution would be to make a 1v1 match. It's best to use the Reinforcements gametype. If you're using any other gametype, you should increase the difficulty so they train more units. Next, pick the Arbiter. The enemy can be anyone you want since it's a Reinforcements gametype. When the match starts, build a Temple and the rest Warehouses. All you should do now is just keep researching the Arbiter's skills and capturing the bases close to you. The bases you capture should all be Warehouses. Once you have all the upgrades for the Arbiter and a fair amount of Warehouses, keep all your reinforcements at your base for protection and uses Arbiter's Rage. You should have a good enough income to keep the rage on the WHOLE MATCH. Once it's on, just keep running around the map killing everything you can. REMEMBER NOT TO GET OUT OF RAGE MODE. You should have this achievement in no time.