You get this when you're 1/1000th of your normal size, and you've dived into the sick Emperor's mouth. You'll go past his uvula, and down his throat.
Make sure you save at the Mirror at the end of his throat. The achievements in this game are temperamental and may not unlock when they should, which is why you should save at a mirror before you're about to get an achievement.
Once you save at the mirror, and go into the next room, you'll face the boss of this area, Blight.
The Boss Fight:
Blight has two main attacks.
The first is him pausing wherever he is, and then charging across the battle area towards you.
The second is sending a spiral of swords at you.
When he's about to charge across the field, he will shake in place. When he's doing this, use Veil of Mist, and when he's crossing, attack him during the Veil of Mist. He'll drop his sword after a while, and you should attack the sword to do damage to Blight.
His spiral sword attack has a glowing blue sword. Use Veil of Mist and attack the sword when it's above Blight. If you do this, he'll drop the sword on the ground, and you can attack it again.
When you've done this a few times, Blight will be defeated, you'll have some cutscenes, and then you'll be controlling the Emperor's body for a minute or so. Follow along the path, and find the girl in the jail cell.
You'll then have a 'results' screen pop up. This is where the achievement will pop.
If the achievement doesn't pop, reload your save file, and try again.