Proving the CCA-insecurity of El Gamal encryption scheme
No, as written, your technique doesn't work (as there's no specific reason why the encryption oracle use the same $y$ as the challenge ciphertext).

You need a different approach.  For one, the adversary doesn't need to depend on an encryption oracle; if he has the public key, then he can encrypt anything he wants.  What he does need to do is use the decryption oracle (as that performs an operation that the adversary cannot do himself).

So, how can the adversary create a ciphertext which is not the same as the challenge ciphertext, but when submitted to the decryption oracle, would tell him the answer?