I think it will be either by Anakin/Vader to solidify Anakin/Vader's skills. If it is Anakin, it will probably be after Anakin becomes Vader and it will signify the only onscreen viewing of the jedi purge. Mace is the most important jedi that doesn't make it to the OT. He would have the most impact on killing as to show the beginning of the purge. The other way I think he could be killed is by Clonetrooper/Stormtroopers. He may be w/ Yoda, Bail,(Obi-Wan?) when Padme is about to give birth or they all are trying to escape on a ship. Mace will try valiantly fend off the troopers so that the others can escape and protect Padme and the twins (born or unborn). Their numbers will eventually overpower Mace, but his efforts allowed the others to escape. I don't think Dooku and definitely not Boba will kill Mace. In fact, I doubt Boba will even be in the movie. What would his purpose be? He will only be like 12-13, so what possible role could he play?