Psycho Delic is the boss of level 6. For the achievement to unlock you just need to defeat the boss without dying. Here is the method I used:
First the point of no return for this fight is when you use the switches to open the large clown mouth to proceed. Again, if you don’t have full health, go ahead and kill yourself so you have a better chance at fighting the boss without dying since you will gain a full life bar upon restarting the checkpoint. If you die during the boss fight the achievement will NOT unlock.
For the boss fight, it is extremely simple. The boss will stay on the top level and you will be attacked by minions and steam on the bottom. There are five buttons on the bottom level that correspond to the steam vents on the top level. All you need to do is run around killing guys and avoiding steam and then press the button underneath the vent the boss is standing on. To make it even easier they even put a big blue arrow on the button you need to press. Repeat 4 more times and he is finished. This whole fight took maybe a minute or 2.