How to parse "就算世界与我为敌"?
与 here serves as a noun conjunction (you could replace it with 和), and links 世界 and 我 forming a composite subject. Its proper translation however depends on the verb that follows, which is 为.
为 oftentimes can be rendered just as "to be". It is after all a copular verb, especially in this sentence, therefore：

就算 / (世界 / 与 / 我) / (为 / 敌)

Even if it's me against the world

However, you should also consider that 与 can work as a preposition, thus it relates to the following 我 more strongly than "pure" conjunctions, so it's also acceptable to parse it as: 
世界 / 与我
For this reason I would tend to give more prominence to 世界 semantically, therefore I think the best translation is:

Even if the world's against me