One possibility if you want to hear is the MC could somehow temporarily mega evolve due to the other legendaria's somehow sending their energy to him and since they are the top power houses in the world something strange like a mega evolve would seem normal to happen. It would allow him to change the size of his sword/dick to be bigger than the one he is imitating by a large amount since that part of him he has the most practice changing while his three basic moves could be enhanced to be able to handle the raw power of etenatus when used correctly. IF mega evolve seems to much the MC could accidentaly find a spot on her like Lugia but different like having her breasts licked though I dont know how he would reach from down below. Also since etenatus did originally come from space she and Deoxys (if collected) could have a scene where enternatus understands what deoxys is saying could explain some of the things she has been saying to the player could just go back to that part of the game and have a somewhat better understanding or at least a scene of them working together on the MC.
