If you have a difficulty finding the way to the final battle, let me walk you through one more time. Once you reach #17, you'll be fighting the final boss Zeromus. 

*** Dark Matter is the key to unlock superboss Proto-Babil,so make sure you steal it from Zeromus.

Boss: Zeromus
Strategy: first battle is kinda scripted because you can't deal any damage to the boss unless you use the Crystal (item), after that he will reveal his true form and you'll be able to deal damage to him. 

*** IMPORTANT: once you defeated the final boss, wait until you give an option to save your game, choose a different slot so that you can continue a New Game+.