This cannot be completed on casual. It has to be done on Classic NORMAL you can do the steps on casual for practice (I heard) but you cant go into the boss!
I made this video for everyone. Remember never do a easter egg unless you know the map really well. I highly recommend doing this with more than 2 players depending on your skill level. If you have any questions leave a comment and I will get back to you. IF YOU NEED HELP PLEASE CONTACT ME AND I WILL HELP YOU.
I wish everyone luck on this easter egg! Its not as hard as everyone says it is with my recommended loadout. My perk order I recommend instead of what I have in the video is dying wish, timeslip, stamin-up, and phd slider (due to the rocket's splash damage). Upgrading the rocket so you can use attachments can be beneficial as well. Also pick up the time slip perk before you pop the firesale so you can hit the box a lot more times.