Halo Wars

After making your way past the Brutes in the beginning, set up your Elephant where your base WOULD spawn. Lock down your Elephant, and set your Rally Point to the BOTTOM Captured Elephant. Then, here's the fun part. Take a brake. I suggest you watch some TV, or take a shower, or go make a Twitter account, something along those lines. Allow your resources to pile up. Once you get a good amount, (It'll take 10,000 to make 100 units, so I suggest you get there.)start sending the marines in. They will have to walk far to get to their destination, so they will be spaced out enough. There's PLENTY of things to kill your UNSC buddies down there, so don't worry about them beating up the nasty Flood.

I wasn't paying attention when I got mine, and I didn't realized it popped up, so if you're taking a break and spawning at the same time, you may want to check your Achievements after a while, to make sure you don't already have it.

get to the point where u get the first base (rescue first elephant) then build all supply pads and send the marines 1 at a time to the other base

Do this on Legendary.

Start, make the waypoint in the middle of the base will all the brutes, and just keep training all the marines and flamethrowers you can. They will run in, get killed, so it will give you more population to build more.

DONT upgrade them, or it will be more likely they will kill the brutes.

NOTE: Each Marine squad counts as ONE Infantry. So you'll need to make 100 marine squads.

i basically did what Killer5079 did, but i had two elephants with spartans in them (i also had the skulls bountiful harvest and pain train on so that it would be faster) also try to put the elephants near the place where the enemy usually comes so that they could get some veterancy and the units will spawn a lot faster, and the spartans help a lot also. now just put down a global ralley point by the enemy flood and let your poor soldiers die. (and make sure you dont upgrade them if you can, so that they die faster)

I did this on co-op, what we did is I would spawn marines (do not spawn flamethrowers they are too good) and transfer control of them to him. He would make them go one squad at a time over to the tentacle things that were guarding the third elephant. We kept doing this so the population would go down (because our current maximum was 30). We kept doing this until we got it. (NOTE: we recommend doing this after you have two elephants (one from the start of the mission and one rescued) so you can have the highest possible population without fear of completing the mission)

here is my video guide for this achievement. Just thought I'd make a video with in depth commentary so you guys have the best idea of how to get this! Feedback and requests are awesome, thanks guys!