How can I break this block-cipher?
Since you only ask for help on your class assignment, I will not give you the whole answer, but will hopefully point you in the right direction.

You are able to decrypt any ciphertext you want, but only have time to decrypt 128. Let's think about this from another perspective. What if you already knew exactly what ciphertext you wanted to decrypt. Say the ciphertext is 126 zeros, followed by two ones (I'll denote this mathematically as $0^{126}11$). You could just ask for the decryption of that, easy enough. 

How could you get the decryption of it, given the decryptions of $0^{127}1$ and $0^{126}10$?