Defenders of Ekron - Definitive Edition

First, you need to beat the main game, and invaders of ekron through level 4. Now go to the music player. You'll see a song called IOE - Level 5. Start it up and exit out of the music player. After a while your screen will go unresponsive and you'll see the boss rampaging in the background. You'll load into a quite difficult boss battle. After beating it you will unlock the achievement.

From the main menu, you need to choose Invaders of Ekron and complete the 4 stage retro vertical scrolling shooter/shmup. I've completed this two times and did not earn the achievement. I am pretty sure this achievement is currently glitched and I've reached out to the developer who are looking into the issue. If anyone else wants to report their findings to the developer as well, please contact them on Twitter http://www.twitter.com/invitrogames