How to Start Mission 21
You’ll receive Mission 21 (along with four other missions) after speaking to Cogita as part of the postgame mission “The Researcher of Myths”. This mission is basically a repeat–and the conclusion–of the lake trials during the endgame. You must visit the three lakes of the Hisui region and catch the Legendary Pokémon there.
1. Head to the Island at Lake Verity
To catch , make your way to Lake Verity, which is located on the west side of Obsidian Fieldlands. We suggest entering via the Heights Camp (otherwise, fast-travel there), then gliding west on . Aim for the circular island in the center of the lake, then go inside the south-facing cave entrance.
Inside, Mesprit will immediately aggro. Throw a Pokémon at/near it to begin a battle.
Dark-types are highly recommended, to stop Mesprit using Extrasensory and Mystic Power, which boosts its offensive or defensive stats, depending on whichever is strongest. This will leave Mesprit with Swift as its only attacking move.
Surviving Mesprit is fairly simple; the tough part is Mesprit using Recover to keep restoring its HP. If you have access to moves that can paralyze or cause drowsiness (eg. Thunder Wave, Hypnosis), use them to try and stop Mesprit from acting.
Using False Swipe will be difficult here, what with Mesprit spamming Recover. Don’t worry about accidentally KOing Mesprit–if that happens, it will instantly respawn.
2. Head to the Island at Lake Valor
To catch , head over to Lake Valor, near the north-east corner of Crimson Mirelands. You could fast-travel to the Brava Arena, which is higher up, and glide east-south-east towards the lake. Or you could depart from the Diamond Settlement instead, which is closer but less exciting. Either way, land on the island in the lake’s center and enter the cave.
Inside the cave, Azelf will immediately aggro. Throw a Pokémon at/near it to begin a battle.
As before, you’d do well to bring a Dark-type. Like Mesprit, Azelf can use Extrasensory and the lake guardians’ unique move Mystical Power, which boosts its offensive or defensive stats, whichever is better (offensive in this case).
Compared to Mesprit, Azelf is much easier to catch because it won’t use Recover. Instead, it can use Double Hit to temporarily boost its move damage by 50%. This will make its Swift hurt harder, but we’ll happily accept this trade off.
3. Head to the Island at Lake Acuity
To catch , travel to Lake Acuity at Alabaster Icelands. You could fast-travel to the Pearl Settlement and use Sneasler to climb up the northern cliff. When you reach the top, glide or swim west to the island in the middle of the lake. Then step inside the cave you previously visited.
Inside, Uxie will spot you immediately. Throw one of your Pokémon at/near it to initiate a battle.
Again, Dark-types are ideal for this fight. Otherwise you’ll be assaulted by STAB Extrasensory and Mystical Power. Compared to the other guardians, Uxie excels in defense, so it won’t be sweeping your party. However Mystical Power will bulk up its defensive stats, making it a slog to weaken.
To further annoy you, Uxie can use Hypnosis to induce drowsiness, which will occasionally prevent your Pokémon from acting. If you want, you can cure it with Full Heals etc. But if you’ve already gotten Uxie’s HP down to red by that point, feel free to let your Pokémon nap.
After successfully capturing the third lake guardian, you will receive the Draco Plate. This mission will then end.