The creator said in a post that it's only available in a bad end for now, there will be a way to get it permanently but not right now.
Get captured by Mistyra at the deserted base and intentionally don't escape. But, you will need the Patreon version of the game to see what happens next.
Outside of that, there's a small window of opportunity to see the mega growths with the free game.
During the Linda rescue sequence, as long as Mistyra is in your party, losing to the common enemies will lead to her messing with you while you're unconscious.