The declarative sentence must be made by a General Council
John of St. Thomas writes:
"[…] This Council can be convened by the authority of the Church which is in the bishops or the greater majority of them; the Church has, by divine law, the right to separate herself from a heretical Pope, and therefore she has all the means necessary for such a separation; now, a necessary means itself (per se) is to be able to legally prove such a crime; but we cannot prove it legally unless if there is a competent judgment, and in such a serious matter, we cannot have a competent judgment except by the General Council, because it is about the universal head of the Church, so much so that it depends on the judgment of the universal Church, that is to say, of the General Council.
I do not share the opinion of Fr. Suarez who believes that this can be treated by Provincial Councils; indeed, a Provincial Council does not represent the universal Church in a manner that this case can be treated by such authority; and even several Provincial Councils have no such representation or authority.
If this is not about the authority under which one must judge, but about one who has the authority to convene the [General Council], I believe that this is not assigned to a specific person, but it can be done by either the Cardinals who could communicate the news to the bishops, either by the nearest bishops who could tell the others so that all are gathered; or even at the request of princes, not as a summons having coercive force, as when a Pope convenes a Council, but as an "enuntative" convocation that denounces such a crime to the bishops and manifest it in order that they come to bring a remedy. And the Pope cannot annul such a Council or reject it because he is itself part of it (quia ipse est pars) and that the Church has the power, by divine right, to convene the council for this purpose, because she has the right to secede from a heretic."
http://www.dominicansavrille.us/on-the-deposition-of-the-pope-part-1-of-2/