Defend stadium was a mission in Jak II. According to Samos Hagai the rift ring was within the Metal Head nest, but to get there they first had to move the heavy rift rider onto Brutter's transport balloon. Which in turn required Jak to defend the two sages while they moved it.
The last escort mission in the game involves the two green sages lifting the rift rider and moving it from one end of the Stadium section to the other. They're otherwise defenseless and can only rely on you, who will have to fend off a large amount of Metal Heads. There are stingers coming from the grassy areas to the left, hoppers out of the fountains on the right and grunts coming down from the stairs in the right. All of these will also come up from behind as well.
You'll be able to take any extra ammunition from the crates at the start before any metal heads appear. But when they do there's going to be a lot of them coming from all sides. You'll be forced to stick as close to the sages as possible (preferably behind them) while dishing out damage with all your Morph Gun weapons. The Scatter Gun is great for anything close by, while the Blaster's aerial wastelander move is excellent for shooting metal heads down all around you.
Use the mini-map to figure where more metal heads are coming from. Finally you should use the Vulcan Fury and Peace Maker when you're about to be overwhelmed, as these make short work of all the enemies easily.