Solid Hide can be obtained by killing these monsters

Guenon 

Lv. 231

Mocus Jungle

Guenon 

Lv. 232

Mocus Jungle

Ronoroule 

Lv. 216

Aselie Special District

Ronoroule 

Lv. 217

Aselie Special District

Urko 

Lv. 218

Draftol

Blagat 

Lv. 239

Yupihi Deep Forest

Blagat 

Lv. 240

Yupihi Deep Forest

Cordil 

Lv. 270

Baiz Road

Cordil 

Lv. 271

Baiz Road

Vasaguille 

Lv. 366

Game Board Cave

Solid Hide is used in the following blacksmith recipes

Additional

Heavy Plate (Upgraded 1)

Spina: 200,000

Claws

Savager

Spina: 2,000,000

Canes

Spell Book Vol. 3

Spina: 250,000

Additional

Totapon (Upgraded 1)

Spina: 2,000,000