When you arrive at Professor Rowan's lab in Sandgem Town, it turns out both he and Lucas/Dawn are relieved to see you alive, as they had been trying to find you. Professor Rowan will mention that the Lake Guardians have returned to their homes. Seeing the number of Pokémon you have, and your seven Badges, he suggests that you head to Sunyshore City where the final Gym is and eventually challenge the Elite Four to broaden your horizons.
You are now free to continue east through Valor Lakefront to access Route 222 and Sunyshore City, but let's visit the Lake Guardians first, as they need to be encountered for you to get the National Pokédex.
To get the National Pokédex, you only need to see the Lake Guardians. If you still want to catch them, then your Pokémon should be at reasonably high Levels, as the Lake Guardians are at Level 50. It's recommended you bring a Steel-type with you to resist the Lake Guardians' attacks, as well as False Swipe or a sleep-inducing move to make them easier to catch. To catch Azelf and Uxie, you should bring about 40 Dusk Balls or Timer Balls. (Timer Balls become more effective than Dusk Balls after 25 turns) Also, you'll need the HM Surf to get across each lake. As always, SAVING before encountering them is a good idea.
When you arrive at Lake Verity, Surf over to the southwestern corner of the lake to find TM38 (Fire Blast). After that, Surf to the cavern in the center and talk to Mesprit inside. It quickly leaves, but is counted as seen in your Pokédex. Professor Rowan shows up and after you listen to him babble about his research, he mentions that Mesprit is likely playing with you and you'll need to track it with the Marking Map app on your Pokétch, which can track roaming Pokémon. If you didn't get it yet, Fly to Jubilife City and go to the Pokétch Company building. Talk to the company President inside. This is the second app you'll receive from him anytime after collecting your third Badge.
Mesprit roams Sinnoh, moving every time you enter a different town or Route. On the first turn of battle, it will flee. This mission will take either patience or a Master Ball. As long as your lead Pokémon is at or below level 50, Repels can make it easier to find in the grass.
Jubilife City is a good place to use for moving between Routes to draw Mesprit closer, as it has four different exits. The guardpost leading to Route 218 or the cave that leads to the upper area of Route 204 are helpful if Mesprit makes it to those places and you are in the right position, since moving through them does not count as changing locations. You can also try moving back and forth across a boundary between two routes until Mesprit appears on your route.
One method to catch it without a Master Ball is to Paralyze Mesprit or put it to sleep with a faster Pokémon the first time you encounter it. On subsequent encounters, attack it to carefully cut down its HP and soft-reset in case you knock it out. Once you've had enough battles to cut its HP bar down to the red zone, throw Quick Balls each time you see it until you catch it.
The second strategy is to use the escape-preventing moves Block or Mean Look, then continue into a battle similar to the one when you caught Giratina. Once you have it trapped, throw Dusk Balls (if it is nighttime) or Timer Balls to catch it. Out of the three Lake Guardians, Mesprit is the slowest, but it has better HP and more balanced offensive and defensive stats compared to Azelf and Uxie.
Now that Team Galactic is long gone, Lake Valor has been fully restored. Surf to the southeastern corner of this lake to find TM25 (Thunder). Then, enter the cave and save your game before talking to Azelf.
Azelf is the fastest Lake Guardian and has the best offensive stats compared to the others, but it has the weakest defenses. With Nasty Plot, it can quickly become a serious threat by boosting its Special Attack. Steel-types are good Pokémon to use because they resist Azelf's attacks. Paralyze it or put it to sleep, then carefully weaken it with a not-too-powerful move. Once its HP is in the red zone, keep throwing Dusk Balls until you catch it.
Uxie in terms of base stats is the opposite of Azelf, where it is the strongest in its defensive stats and weakest in its offensive stats. It can use Amnesia to raise its Sp. Defense stat by 2 levels. Use non-supereffective physical moves to lower its HP and then use False Swipe if necessary. Uxie may try to put your Pokémon to sleep with Yawn which can be an annoyance until Uxie's HP is in the red zone and you can focus on catching it. Just like with Azelf, Dusk Balls are the best way to go, though Timer Balls may be needed as backup.
Now that you have dealt with the Lake Guardians, Fly back to Pastoria City, then head to Hotel Grand Lake at Valor Lakefront. Time to travel east to your final Gym battle!
Route 222 is the path from Valor Lakefront to Sunyshore City. It's worth noting that it's also the only location where you can find a wild Electabuzz. The two houses on the beach are not essential to visit, but the one on the left is a Pikachu fan club. One of the Pikachu inside is actually a Poké Kid who will battle the player using two Pikachu of her own! The other house has a Fisherman inside who is looking for a giant Remoraid that is larger than 23.7 inches. If you show him one then he'll give you a Net Ball.
Also on this route is the final Honey Tree you'll find in the game. The man at the end of the cliff will give you TM56 (Fling). If you ever use Fling in battle, be aware that usually the user's held item will be lost forever upon use (some places where it's kept include the Battle Frontier and player-versus-player battles). Make sure to battle the Fishermen on the beach, as one of them has a Feebas, which is a hard to spot Pokémon that you will need to see to complete your SinnohPokédex. Once you are done here, head through the gatehouse to reach Sunyshore City.
When you enter Sunyshore City, you'll encounter Flint of the Elite Four, who asks if you can try to cheer up his friend, Sunyshore's Gym Leader Volkner, who has been bored lately from the lack of excitement. You can explore around the city if you want, but before you can challenge the Gym you need to find Volkner at the lighthouse. Also, on the beach in the northern area of town, you can encounter Jasmine, a Gym Leader from Johto. It's most important to talk to her after you beat Volkner, though, because she will then give you HM07 (Waterfall).
In the house on the cliff on the eastern side of town, accessible through Rock Climb, is a man that will give you three new Pokétch apps based on the Nature of your Pokémon that are in your current party.
If you have a Pokémon with a Serious Nature inside your Party, you will get the Calendar App.
If you have a Pokémon with a Naive Nature inside your Party, you will get the Dot Artist App.
If you have a Pokémon with a Quirky Nature inside your Party, you will get the Roulette App.
The right-hand house in the northeastern corner of the city belongs to a woman named Julia. Visit her daily to obtain a Ribbon. The Ribbon you receive is dependent on which day of the week it is. She does not give out a ribbon on the first visit.
The large building in the southeastern corner of the city is the Vista Lighthouse. First, use the Dowsing Machine to find a hidden Zap Plate here. Inside, talk to Volkner and you'll leave him convinced to accept your challenge at the Gym.