Have you considered Protect on the last slot? It could be like Sub/BU/DTail/Protect, much like the Kyurem set that terrorized UU at its conception (Sub/HC/DTail/Protect). Dialga has better typing and defense boost, so maybe it could work?
Idk I'm not very sold on this set because it doesn't seem to do enough damage for its lifespan. Maybe if you could keep it alive with Wish...idk. Some logs would be nice.