"carf"? :)

If you want the opponent to chose:

> "Each player who milled a creature card this way chooses a creature they control. It gets -3/-3 until end of turn."

If you want the caster to choose, you need a reflexive trigger:

> "When a player's mill a creature card this way, target creature that player controls gets -3/-3."

If you want the opponent to choose, but insist on targeting anyway, use a reflexive trigger:

> "When a player's mill a creature card this way, target creature that player controls of their choice gets -3/-3."