I finished all the side quests besides the last two and that Spiritomb one
Major spoiler for postgame- I am having a hard time against Volo. I beat Giratina with my last Pokemon but then it changed form and gg. Need to do it all over again. Tried leveling up against some people on the training grounds but it's soo slow
I decided not to grind and instead used my level 85 alpha Garchomp (you can find it in the icelands). Strong style Outrage from him will probably not kill either Giratina but will blow a big enough hole in it. You can also use an aux power and absorb a dragon claw.
My advice would be to exploit his Pokemon that can't hit you too hard and use them to heal. For me that was his Roserade, which could only 3hko Goodra. Heal everything up when you're facing that so you're ready for Giratina.