First, I recommend not using Giratina at all. You already have Garchomp on your team, and Garchomp is a dragon Pokemon that has stronger physical attack and speed and gets STAB on earthquake. The only situation when Giratina is better than Garchomp is when you're using HM moves or battling Lucian. In the second situation, you can just teach calm mind and shadow ball to Gardevoir and have Gardevoir completely solo that battle. So Giratina can learn HM moves and not do anything else a lot better compared to your other team members.
If you really want to use Giratina, replace Garchomp. Both Garchomp and Giratina have slow leveling rate, so keeping both of them will make your other team members gain less experience (unless you never let Giratina gain experience and use it as a HM slave). Garchomp and Giratina are also both dragon type, so keeping both of them will give you a harder time against any strong opponent (such as Cynthia's Milotic and Garchomp) knows an ice or dragon move. Replacing any other team member makes you lose more type coverage.
