Halo Wars

Another method to this achievement is much easier and faster. The game should be a 1v1 skirmish match with you using Arbiter, and the A.I. using anybody set to normal difficulty. For this method to work you must be in a death match. The first thing to build is your temple because the sooner you have him attacking the better. Fill the rest of the spaces with shield generators in case you are attacked while you are gone. By the time you get the achievement you should only be attacked once or less. As soon as Arbiter is out, send him to the first extra reactor in rage mode. Destroy all the enemies there and continue to the next reactor on the map. Make sure to stay in rage mode. You are using arbiter and are able to run freely around the map without stopping your rage. Since you are playing on a death match, you already start with a ton of resources and an upgraded leader. Destroy all the units on all of the extra reactors scattered on the map and then head to your enemies base. If you did this quick enough, he should have only a hand full of units, but that is all you need for this achievement. Note: You may not want to do this against a UNSC because of the possibility they will drop a disruption bomb, but on normal difficulty you may not have that problem.

My solution to this would to make this 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.

Another alternative to all of the solutions posted here is the one that I did. Perhaps not as timely as the others, but the Arbiter broke his leader power a couple of times while I was going for this achievement.

Play as the Brute Chieftain with easy/normal A.I. in a 1v1 match. Build 10 Blessed Warehouses, fully upgrade your Brute Chieftain, and, preferably, build 2 Engineers.

Take whatever you have a clear a way for your Brute Chieftain all the way to the opponent's main production base and put the Engineers with him. Start up your power and place the Vortex right outside of the exit to your opponent's base.

Now, every unit he builds will come out of the base straight into your Vortex, the Vortex will gain strength with each kill, the Engineers will make sure that your Chieftain doesn't die, and the abundance of supplies coming in from your warehouses will have you still gaining supplies regardless of how long your leader power is active.

I set this up, realized how well it worked, left my room to get/eat some pizza, and by the time I came back, I had unlocked the achievement with a Chieftain still at full health and a Vortex that I nudged over the their base and destroyed in 10 seconds.

**Check Note at the bottom before starting.

For this, you must K.O. 20 Units or more with one of the three Covenant Leader's Power in one activation. You can use the Arbiter with his Rage, the Brute Chieftain with his Vortex, or the Prophet of Regret with his Cleansing Beam (I personally chose the Prophet so I'll explain it for him, but I've seen the other leaders chosen as well).

The guide to this is either you can start up a 1v1 against a computer AI and go against the odds by hoping that it'll cluster 20 units together (which will never happen) for you to kill or you can boost with a buddy (I'd suggest the latter method). So start up a game build a Temple and four Warehouses. Now while that's going on, have your buddy build 20 weak infantry units, such as the Elite Honor Guard (Covenant) or Marines (UNSC). Now all your job is, is to upgrade the beam to as far as it can go (meaning you'll have to upgrade your Temple twice). Your buddy's job is to send all 20 of his units to a specific location determined by you and once you have enough money (several thousand to be safe) go over with your Prophet and get fairly close to them (as to avoid being out of range and restarting the 20 consecutive kills). Now activate your beam (press the "Y" button) and move it around with the Left Stick (targeting units as you go). Once you've killed all 20 and it was successfully done in one go (meaning that you didn't stop and start the beam once), you'll be 30G richer and be 25% done with the new achievements for Halo Wars!!

**Note: Two things to add, if you're buddy builds flamethrowers and you upgrade your Prophet to "Divine Absolution" where he can fly, you wont have to worry about taking potentially harmful damage.

Also if you want the process to go faster you should expand your first base to a Citadel and build two more Warehouses and then maybe even grab the base next to yours and add three more Warehouses. (ALWAYS BE SURE TO UPGRADE ALL WAREHOUSES TO BLESSED WAREHOUSES - You'll be done with this one so much faster).

-One more thing I found using this method, the person who's building the flamehtrowers, don't touch them after they're built, because they'll all gather near the base rally point as close as they can and it's hard to gather that many units into such a small space when they have already dispersed, even if you individually move them back one at a time. This way you'll be able to hit them all in one go with ease as they are so close together.

Have Fun!

You can do this one by yourself by playing 1v1 on Deathmatch. Play as the Prophet and take all of the other bases. Make all of the bases are supply bases with one of the slots being the temple. Wait until you have 26,000 resources then send your prophet to the entrance of the AI's base and activate the special ability. Kill any units that come out but do not end the stream. Your resources will go down very slowly. like 1,000 every 5 mins. This should be plenty of time to kill 20 units.

Get this and Never Leave a Man Behind in one go.

Start a standard 2v2 game on Memorial Basin, easy difficulty, with you as the Arbiter, your teammate as anything, and your opponents as random Covenant.

Start by building a warehouse and queue up a temple and another warehouse, and send your Ghost around to pick up resources. As your warehouses are built, upgrade them to Blessed Warehouses. Take two more bases (one next to you, one next to your ally who will be unlikely to take it himself), and fill them with Blessed Warehouses. You may also clear out a nearby reactor and staff it with a unit of Suicide Grunts (which you can build from the base core without building a Hall), which will save a little time. Along the way, spend extra resources upgrading your leader abilities to maximum. Fend off attacking armies from you and your ally as necessary.

Once your abilities are maxed, wait until you have just over 5000 resources. Select a warehouse and donate all of it to your teammate. Now, select the Arbiter and go into Rage mode. Walk around the map taking out enemy units and reactor guards until the Drain Cleaner achievement pops. Do not exit Rage until this happens -- you lose no resources by walking, and what you do lose by attacking will refill quickly. Because your enemies are Covenant and not UNSC, they will be unable to stop your Rage with a Disruption Bomb. If, after all reactors and enemy units, you still come up short of 20, just walk the Arbiter away (with your left stick; stay in Rage) and hang out until more enemies are built.

Once Drain Cleaner pops, take your Arbiter to the enemy bases and go to town. Two Memorial Basin achievements, one game.

There is perfect guide. I take no credit in the video