For Groudon I'm thinking the really bulky variant with Impish and a loaded up defense could really benefit your team. Try Stealth Rock/Toxic/Thunder Wave with EQ, Stone edge and Dragon Tail. This would allow Dialga to switch Stealth Rock with something else if you run Stealth Rock. If you run Toxic/Thunder Wave you could easily cripple Lugia (who you mentioned as a threat) especially since Groudon easily lures it in. Most importantly though you get another answer to Extreme Arceus as you can phaze it with dragon tail or cripple it. (Since it's running so rampant having more then one answer is ideal).
Edit: I would keep Outrage on Dialga. Having a powerful attack to fall back on once you use DM is always useful and it strengthens your team against Kyogre and Chansey/Blissey.
The most haxxed.