Long story short? It´s me against the world/now it´s me against the world.
"Me" is commonly used when the word or its phrase is in isolation"
(source -http://www.usingenglish.com/forum/ask-t ... onoun.html
I don´t see many cases for just "X....whomever against the against the world"
without the X complement. Generally "X" is "it´s"...
When there are two characters in the place, "You and I" , then there´s no
"me" in isolation, as the site explains. So, "Now it´s you and I against the world", as an example.