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.
Can you "look out sir" direct damage? I cannot remember without checking the rule book. If so a Slann would automatically pass this and could effectively not be targeted.
Just get MR2 and you'll have a 2+ against all wounds caused. Saves your dispel dice for Soulblight and Purple Sun.
This. 30 points makes you largely immune to all of the direct damage. You'll really want to save dice to stop purple sun and soul blight. If you have dice left, then spend the 20 points to turn one of them into a power die. -Matt