It's admittedly a little convoluted, but some of the achievements and the final log you collect give you hints, haha
You first need to have the Moonstone enemy equipped to activate the computer in Cosmic Research (it'll be glowing in the upstairs room), after that, you can find and collect all the logs in the game. After you find the last one in a level, you'll need to find the hidden glowing portal in the Industrial Complex to collect the last log. Do that and now all you need to do is beat every level on every difficulty (doesn't matter what rank you get) and you should be able to encounter the final boss in Death Corn.