The most familiar witness-preserving amplification for QMA is based on Jordan's lemma and uses the projections $\Pi_1$ and $\Pi_2$ where $\Pi_1$ is he projection on the 'ancilla zero' space, and $\Pi_2$ is the projection on the space of accepted states by the original Arthur (for the full proof, read here).
My question is how can I prove the witness-preserving amplification theorem for QMA, using phase estimation of $e^{i\Pi_1\Pi_2\Pi_1}$ ?
I know it is possible (otherwise, it wouldn't be a question in the HW assignment I have received), but I just can't find the solution.
Any help or reference to a helpful source would be appreciated