XIII_rocks posted... 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.
https://imgur.com/a/74et9pTI took that advice to the extreme
Let's so how it will work lol