"The boy's enemy is the girl's friend."
Translation:Guttens fiende er jentas venn.
min is a possessive pronoun and can stand before or after a noun (which is then either indefinite in the first case or definite in the second).
Ex.: min far = faren min
guttens is the genitive case of the definite form of en gutt (= so-called s-genitive), not a pronoun. Hence you need to treat it differently. It has to be before the object that belongs to it.
fienden til gutten is another way to express belongings, the prepositional genitive, which you know from English as well ("the enemy of the boy"). Here, the genitive construct (preposition + noun) needs to follow after the object that belongs to it.