To get the Protector, you need to first complete the national pokedex, which then allows you to leave the fight arena city. Go right, then surf, until you get to the Resort Area. Then go up as far as you can go on Route 228, until you get to the building that leads to Route 226, with an Ace Trainer standing nearby. The Protector should be in a hole that has a "slippery slope" right next to this building.
(Note: Route 228 has level 50+ wild pokemon as well as a lot of trainers with level 50+ pokemon. There is also a raging sandstorm. Bring capable pokemon and some potions to heal the sandstorm damage.)