Cool idea for a team! I think it actually seems to work, and you have checks to most prominent threats. Some stuff I would change:
Lum Berry for Tyranitar, and I'd recommend you to have Stealth Rocks on Tyranitar, because your set is defensive and it frees up the offensive capabilities of Groudon. Lum is great for Darkrai leads too, which is a problem for this team otherwise!
Rock Slide has its merits on Excadrill, but most of the time I prefer Iron Head, as it helps more vs Giratina-O and Latias. I'd not worry too much about not hitting Ho-oh that hard, Ttar usually gets rocks up and can check it.
Grass Arceus is a horrible Calm Minder, but Grass Arceus has great merit on your team, so don't replace it! I just suggest getting rid off Calm Mind for Ice Beam or Thunder Wave here. Both has merits, Thunder Wave is an amazing support more for any team while Ice Beam destroys Rayquaza, which is a bit troublesome.
With SR on Ttar, you can opt for a Rock Polish set on Groudon! Check out some website stuff and see what suits your team the most (I'd use something along the lines of EQ, RP, D claw and Rock Slide). Just beware of a slight problem with checking Extremekiller Arceus if you opt for this set.
Ho-oh can use a physically defensive set to help check Extremekiller and a myriad of other mons, especially if you perform the above changes to your team.. However, it loses a lot of firepower so think through this. Good luck with the team!