Creatures begin life in one form and often find themselves ... you need to evolve it to get your hands on a Bastiodon. All it takes it getting Shieldon to level 30. This is done by gaining EXP ...

Since you will be catching Shieldon above that level, all you need to do is level it up once to make it evolve. This can be done by battling trainers, using EXP Candy, or feeding it a Rare Candy.

Probably the main way to guarantee encounters with Shieldon/Cranidos and therefore increasing your chances of finding their Shiny Forms is by raiding. The duo will be appearing in One-Star Raids ...