Walkthrough: Before heading to the Dimensional Research Lab, you can actually hang back and try to catch some of the many Pokemon in Lush Jungle. Believe this: there are quite few.
Wild Pokemon:
Grasses - Daytime (Levels 18-21): Paras(20%), Trumbeak(20%), Fomantis(20%), Metapod(10%), Caterpie(10%), Bonsly(10%), Comfey(5%), Oranguru(5%)
SOS Pokemon #1 - Daytime (Levels 18-21): Trumbeak, Paras, Caterpie, Fomantis, Bonsly, Comfey
SOS Pokemon #2 - Daytime (Levels 18-21): Trumbeak, Paras, Caterpie, Fomantis, Sudowoodo, Comfey
SOS Pokemon #3 - Daytime (Levels 18-21): Trumbeak, Paras, Caterpie, Fomantis, Happiny, Comfey
SOS Pokemon #4 - Daytime (Levels 18-21): Trumbeak, Paras, Butterfree, Fomantis, Bonsly, Comfey
SOS Pokemon #5 - Daytime (Levels 18-21): Trumbeak, Paras, Caterpie, Fomantis, Bonsly, Comfey
Grasses - Nighttime (Levels 18-21): Morelull(20%), Trumbeak(20%), Fomantis(20%), Metapod(10%), Caterpie(10%), Bonsly(10%), Comfey(5%), Oranguru(5%)
SOS Pokemon #1 - Nighttime (Levels 18-21): Trumbeak, Morelull, Caterpie, Fomantis, Bonsly, Comfey
SOS Pokemon #2 - Nighttime (Levels 18-21): Trumbeak, Morelull, Caterpie, Fomantis, Sudowoodo, Comfey
SOS Pokemon #3 - Nighttime (Levels 18-21): Trumbeak, Morelull, Caterpie, Fomantis, Happiny, Comfey
SOS Pokemon #4 - Nighttime (Levels 18-21): Trumbeak, Morelull, Butterfree, Fomantis, Bonsly, Comfey
SOS Pokemon #5 - Nighttime (Levels 18-21): Trumbeak, Morelull, Caterpie, Fomantis, Bonsly, Comfey
Weather Battles (Levels 18-21): Castform, Goomy, Castform, Castform
Grasses - Daytime (Levels 18-21): Bounsweet(40%), Trumbeak(20%), Fomantis(20%), Paras(10%), Comfey(5%), Oranguru(5%)
SOS Pokemon #1 - Daytime (Levels 18-21): Bounsweet, Trumbeak, Fomantis, Paras, Comfey
Grasses - Nighttime (Levels 18-21): Bounsweet(40%), Trumbeak(20%), Fomantis(20%), Morelull(10%), Comfey(5%), Oranguru(5%)
SOS Pokemon #1 - Nighttime (Levels 18-21): Bounsweet, Trumbeak, Fomantis, Morelull, Comfey
Shaking Plants (Levels 18-21): Fomantis(100%)
Weather Battles (Levels 18-21): Castform, Goomy, Castform, Castform
Grasses - Daytime (Levels 18-21): Paras(30%), Fomantis(30%), Trumbeak(20%), Pinsir(10%), Oranguru(5%), Comfey(5%)
SOS Pokemon #1 - Daytime (Levels 18-21): Paras, Fomantis, Trumbeak, Pinsir, Comfey
Grasses - Nighttime (Levels 18-21): Morelull(30%), Fomantis(30%), Trumbeak(20%), Pinsir(10%), Oranguru(5%), Comfey(5%)
SOS Pokemon #1 - Nighttime (Levels 18-21): Morelull, Fomantis, Trumbeak, Pinsir, Comfey
Weather Battles (Levels 18-21): Castform, Goomy, Castform, Castform
Walking (Levels 18-21): Zubat(70%), Diglett(30%)
Zygarde Cells: There is one Zygarde Cell found here.
Items:
Forage Bag (South) - From Mallow During Trial
Tiny Mushroom (South) - Southwest Jungle (Hidden)
X Defense (South) - Northeast Corner (Hidden)
Max Revive (South) - Small Passgeway
Nest Ball (South) - From Mallow After Trial
TM67 Smart Strike (South) - From Kukui After Trial
X Sp.Atk (West) - In Rock By East Entrance (Hidden)
Mago Berry (West) - Southwest Corner (Hidden)
TM86 Grass Knot (West) - In Small Area By Trees
Revival Herb (North) - West Patch of Grass
Hyper Potion (North) - Small Bush By Sign (Hidden)
Heal Powder (North) - In Rock South of Moss Rock (Hidden)
X Attack (East) - Near Tauros Rocks in Northeast (Hidden)
Miracle Seed (East) - Under Tauros Rocks (Hidden)
Awakening (Cave) - South of Exit
Rare Candy (Cave) - Southeast of Exit
Post Grass Island Trial
Details: After defeating Mallow's Trial, you'll later be able to find her in her home. Eventually following that, you will Battle her in Lush Jungle.
Captain Mallow - Level 26 Phantump, Level 26 Shiinotic, Level 27 Steenee