This isn’t actually as hard as it sounds and will come naturally during the campaign. Swarms of 100s and later 1000s of enemies attack your base every so often with a massive attack triggered every time you do a Headquarters upgrade.
I’m sure this is doable in Survival but I’m not sure a survival map is intended to become quite as epic as Campaign mode. You’d have to hang around longer than necessary.
Anyway, the campaign lets you progress through the biomes somewhat as your choose, but you’ll get this maybe halfway through. Just build a lot of defensive towers and you mostly don’t even have to do the killing yourself.