I was planning on writing up a post for the previous scenario but I lost track of time. I'm still having a hard time believing that it's already the 23rd.
So, the mon that is most likely to succeed against itself? And by mirror match, do you mean the exact same sets?
Hmm... I can't say the I completely understand this scenario, but I'll make a start.
My initial thoughts lead me to consider either Mewtwo or Dialga, since they both have access to strong neutral attacks. Both Dialga and Mewtwo can use Ice Beam or Fire Blast, which are the strongest neutral attacks they have access to (off the top of my head).
Gira-O, on the other hand, relies heavily on its STABs for damage. Gira-O obviously can't use earthquake, and its HP fire will do abysmal damage. So it looks like Stone Edge and Psychic are the only viable options.
Manaphy's only reliable attack is Psychic.
Yeah, I'm not liking this one. Seems kinda busted.