Well, Psycho Killer Mewtwo vs. Wish Support Jirachi
252 SpA Life Orb Mewtwo Psystrike vs. 252 HP / 40 Def Jirachi: 58-68 (14.35 - 16.83%) -- 9HKO at best
252 SpA Life Orb Mewtwo Aura Sphere vs. 252 HP / 216+ SpD Jirachi: 108-127 (26.73 - 31.43%) -- possible 4HKO
252 SpA Life Orb Mewtwo Ice Beam vs. 252 HP / 216+ SpD Jirachi: 57-67 (14.1 - 16.58%) -- 9HKO at best
and Jirachi can simply stall with Toxic / Wish or U-Turn to gain momentum.
Only Mewtwo with Fire Blast can beat Jirachi, but in rain,
252 SpA Life Orb Mewtwo Fire Blast vs. 252 HP / 216+ SpD Jirachi in rain: 143-169 (35.39 - 41.83%) -- 84.5% chance to 3HKO
(this will hit hard outside of Rain though)
Lastly, we picked Dialga for its ability to beat a majority of weather abusers and check Arceus-Ghost, but seeing as we have Darkrai (and hopefully Rayquaza), we'll be able to (possibly) replace Dialga with something more reliable, seeing as the two I mentioned can cover the roles we initially wanted Dialga to cover. Lastly, Jirachi was not picked possibly because it loses offensive momentum, seeing as it has a relatively low offensive presence.