Messages of different lengths and one-time computationally-secret
This is already explained well, at a conceptual level, at a different question: Why is a non fixed-length encryption scheme worse than a fixed-length one?

To turn it into a formal proof, write down an adversary $A$ that breaks the security of $\Pi$.  You may want to first work out the attack at a conceptual level to make sure you are clear on how to attack $\Pi$; then review the definition of what it means for an adversary to break $\Pi$; then turn your conceptual attack into a fully specified algorithm $A$, and check that it does indeed break $\Pi$.