Hmm....OK here's a plan. Before ANY Legendary PKMN Battle, always remember to SAVE THE GAME!!!!! OK, once you encounter Giratina, immediately use a Quick Ball. If that doesn't work, have Mismagius in your lead and start out strong using Shadow Ball (I know you have that move). Giratina should counter by Using Shadow Force/Ancientpower/Dragon Claw. It should be in the Yello point at its Health Bar. After that, switch to Infernape and start chipping away at its HP using moves like Flame Wheel, Fire Blast, ThunderPunch, or FirePunch. If Giratina gets Infernape to the Red point in its health bar, switch to Altaria. Revive Infernape. Keep repeating that method until you run out of Revives or if Infernape is your last man standing. If Infernape is your last man standing, just immediately use the Master Ball but if you catch it with the Master Ball, you have no more Master Ball! You can always win another Master Ball using the Jubilife Lottery but that'll take awhile and TONS off luck. Hope I Helped!