Walkthrough: After defeating your third Island Trial, you can head back down Wela Volcano Park and catch all the Wild Pokemon found throughout. After doing so, head to the tunnel that divides Route 7 and Route 8 to proceed to your next Island Trial.
Wild Pokemon:
Initial Pokemon (Levels 16-19): Fletchling(30%), Salandit(30%), Cubone(24%), Magby(15%), Kangaskhan(1%)
SOS Pokemon #1 (Levels 16-19): Fletchinder, Salazzle, Cubone, Magmar, Kangaskhan
SOS Pokemon #2 (Levels 16-19): Fletchinder, Salandit, Cubone, Magmar, Kangaskhan
SOS Pokemon #3 (Levels 16-19): Fletchling, Salandit, Kangaskhan, Happiny
SOS Pokemon #4 (Levels 16-19): Fletchling, Salandit, Cubone, Magby, Kangaskhan
Trainers:
Sightseer Mariah - Level 19 Meowth
Ace Trainer Jim - Level 20 Noibat, Level 21 Kadabra
Hiker Calhoun - Level 19 Roggenrola
Hiker David (Kiawe's Trial) - Level 19 Magmar
Items:
Hyper Potion - Grasses Near Route 7 Entrance
Antidote - Near Trial Site
Float Stone - Halfway Down Mountain
TM39 - Near Ace Trainer Jim
Burn Heal - On Cliffside Down Mountain