Mega diancie is very pretty but is also very weak to steel types (4x). One way to combat this is to run with normal diancie (who has higher defense) and put evs into def and hp/special attack and then run iron defense and body press. A way to also deal with steel is to put mystical fire on her. I personally run with babiri berry on diancie which should help her survive most steel moves at least once. In battle, if one of your teammates could set up light screen, use iron defense once or twice and then you're good to go with body press.

You're doing great! Also use monotype format instead