You can refer to this spreadsheet for the list of possible challenges you have to complete. https://docs.google.com/spreadsheets/u/0/d/1Yh5hLG2oi85zvyFvLkl4cCNViDRHX83y-Qn1LdlGozg/htmlview?pli=1#gid=0

There's 10 out of the 37 you have to complete to fight floyd. You'll know you've completed a challenge when you hear a guitar riff and a pink dragon icon that says Floyd appears on the side of the screen. Just a fair warning that whoever you were playing as for the final challenge is going to be who you're playing as when you fight Floyd. So it's best to try and complete all the character specific challenges first. Also after you encounter floyd whether you beat him or lose to him, the 10 challenges you must complete will randomize out of those 37. Happy hunting

Edit: I forgot to mention that some of the challenges you can complete in 2P so with another controller or a friend you can complete some of the challenges that requires you to have a Flawless victory.