Just to be clear, there aren't any Magic Missles in the Lore of Death. Also, Magic Missles do not allow targeting single models within a unit. There are, however, a few Direct Damage spells in the Lore of Death that allow targeting "...a single enemy model (even a character in a unit)." Spirit Leech, The Caress of Laniph and The Fate of Bjuna all have this rule. Also note that these spells do not require line of sight. However, the target must lie within the caster's forward arc, per the Choosing a Target on pg 31 of the rule book.