You have to defeat a Boss without hitting their Biometal areas in order to get a Level 4 victory and unlock a toy version of that Boss in the back of Prarie's room:
Fistleo: beat Fistleo but don’t hit his head.
Flammole: beat Flammole but don’t hit his arms.
Hivolt: beat Hivolt but don’t hit his wings.
Hurricaune: beat Hurricaunce but don’t hit her neck.
Leganchor: beat Leganchor but don’t hit his head.
Lurerre: beat Luererre but don’t hit her head.
Protectos: beat Protectos but don’t hit his chest.
Purprill: beat Purprill but don’t hit his arms.