SOUTH PARK: SNOW DAY!

Invisibility is gained through one of the Bullshit cards you select at the start of your run. If it isn't available in your selection, just keep restarting. It has 2 uses so make sure to save them for the end of the run.

I found Cartman the easiest boss to do this on. He doesn't fly or use many attacks that push you away meaning its easier to get the timing right between using the card and landing the final blow.

Cartman can be fought at the end of Chapter 4. It's a relatively simple boss battle as he stays on the battlefield for a majority of the fight and is a lot slower than the others so you can reach him with ease. He also uses a bubble shield a lot meaning your AI companions are less likely to take him out using a ranged attack. The snow decoys he makes is also good for keeping them distracted.

Get his health down to the final bar and use the invisibility before your final attacks. Just be mindful if you are using the daggers as they may cause him to take bleeding damage. I was able to time my final strike in between the bleeding to make sure it worked.

This can be achieved on easy difficulty if you are having difficulty avoiding his attacks, and Dark Matter perks such as increased melee damage make it a lot easier to drain his last bit of health in time.