Eh, I've done some testing with
Keldeo > Forretress, and Keldeo fits your team like a glove. Keldeo allows you to easily beat Blissey and Chansey, who were giving you quite a hard time before. Keldeo is also a great revenge killer in the Rain, and offers you some much needed speed for this team. I replaced Perish Song on Celebi for Psychic, and I made Politoed a defensive set.
HOWEVER, I feel that
Dugtrio is very underwhelming. I'm sure it might help you when facing sun teams, but I've had roughly 20 battles with this team, and I haven't run into one sun team. I also feel that Dugtrio is terrible as an SR user.
I feel the best solution for this is
replacing Dugtrio with support Terrakion. Terrakion sets up SR very well, helps you loads with Sun teams, and in general is just a great Pokemon.
Here's the set
Overall, its one of my favorite teams to use, and I hope my suggestions helped. GL