It says something about Giratina that the God of Time is bat-shit terrified of it.
Zero's rampage in the Reverse World causes the mountain glacier to move, destroying everything in its path. The Pokémon in the valley try their best to stop the Advancing Wall of Doom, but to no avail. As the glacier breaks apart, we cut to a small temple hidden deep in the mountains, where the legendary Regigigas slumbers. The giant awakens, and sensing that his domain and everything living in it is in peril, moves out and, together with a herd of Mamoswine that he summons, holds the glacier back. They weren't able to completely stop the glacier, but they bought enough time for the heroes to undo Zero's damage.
It's real awesome to see all the Pokemon, including a just-transformed Sky Forme Shaymin, fighting Magnezone and his army of Magnimites on the train.