The guide made by AbsolutioN XI does not work anymore, so I have made an updated guide with a new skip.
First thing's first, make sure you have an item equipped so your game doesn't crash and so you don't lose your mind. You can have items equipped, you just can't use them. If you don't have an item equipped your game will crash right at the end of the last level.
Let me start by saying that this solution will only cover the last level. If you're going through one of the other levels and are thinking "this is impossible" well then baby, you ain't seen nothin' yet. Passing these levels by yourself will be good skill building if you ask me because every time I passed a seemingly impossible level, I not only felt extremely happy but felt like my skills in the game grew by a lot. So if you're having trouble with earlier levels, just keep at it, you'll "get your lucky break" to quote AbsolutioN IX (the other guide poster). Now for the actual guide, I must thank Absolution because without his skip, I feel like like I wouldn't have found this one.
As far as I know, no-one else has found this skip, but I may be wrong (I searched everywhere when feeling like giving up on this achievement). After the video ends, you pretty much just climb him again, land in the same spot, climb him and you're done. I believe you have no time limit when climbing him after doing the skip. When jumping on the truck repeatedly, you want to keep jumping until you hear the boss stop breathing fire.https://screenshotscontent-t4002.xboxlive.com/000900000b066a...
Be wary of phallic objects.
If something in my solution could be done better, feel free to let me know in the comments so I can make this solution better! Good luck!