UN Resolution 3314 is not binding, and Art 3(g) is "just a recommendation". Whether or not A can bring a claim against B has to be determined by the laws of a country with jurisdiction or, possibly, an international law. The summary is that an individual has no standing to pursue the matter in ICC; I don't know whether individual A could bring a claim against B in Germany under German law.
The crime of aggression is defined by an amendment in 2010 (the Kampala Amendment) to the Rome Statute of the ICC. As it happens, the amendment was
accepted (not ratified) by Germany. The crime of aggression is defined in the amendment as
the planning, preparation, initiation or execution, by a
person in a position effectively to exercise control over or
to direct the political or military action of a State, of
an act of aggression which, by its character, gravity and scale,
constitutes a manifest violation of the Charter of the United Nations
Then there is a long list of conditions defining "act of aggression". The Rome Statute has numerous amendments, with the definition of aggression being the only relevant one that is in force. Now we can look at the statute itself (the originally passed statute). As a preliminary, "The Court has jurisdiction only with respect to crimes committed after the entry into force of this Statute" (Art.11) Under Art. 12, if a state becomes a party to the statute then they are subject to the court's jurisdiction w.r.t. the claim. Germany could then request an investigation by the prosecutor (and could be investigated), under Art. 14:
A State Party may refer to the Prosecutor a situation in which one or
more crimes within the jurisdiction of the Court appear to have been
committed requesting the Prosecutor to investigate the situation for
the purpose of determining whether one or more specific persons should
be charged with the commission of such crimes.
There is no limitation to the effect that the state party has to have been in any way an aggrieved party. Art 13 gives the court jurisdiction if
A situation in which one or more of such crimes appears to have been
committed is referred to the Prosecutor by a State Party in accordance
with article 14...A situation in which one or more of such crimes
appears to have been committed is referred to the Prosecutor by the
Security Council acting under Chapter VII of the Charter of the United
Nations or...The Prosecutor has initiated an investigation in respect
of such a crime in accordance with article 15
So the bottom line is that Germany can request prosecution, or the Security Council can, or the Prosecutor can sua sponte prosecute an alleged crime. But Mr. Jones has no standing in ICC for alleged aggression.
An alternative is that maybe Mr. Jones can bring a claim against an aggressor (let us say, the Elbonian Empire) under German law, but that would be orthogonal to questions about the Rome Statute.