This can be unlocked by defeating the PHANTOM Commander boss on Level 18.
To start off dodge his fireballs and green spheres until he hovers his fist above you and slams it down. This will create a shockwave so dodge that and attack his hand at this point. He will repeat the process so follow suit.
When his arm breaks off he will attempt to headbutt the ground. This will also create a shockwave so dodge this phase 3 times and he will damage himself. He now begins to use his remaining good arm so follow the previous section until it also comes off.
In the next phase he spits out bombs with a tennis ball in the mix so dodge the bombs and uppercut the tennis ball back to damage him. During this he will be firing homing missiles and laser beams. For the missiles run left (or right depending on what side of the platform you are on) and jump over the volley when the get low. For the laser beams you can just jump over them as it's only the place where they impact that it does damage to you.
For the final form (a floating head) it will be mostly the same as the previous armless phase, just that he attacks MUCH faster now. Knock the tennis ball back at him 3 times to defeat him permanently.