Ridden monsters

The FAQ applies only to "spells that pick out individual models, even if they are in a unit." Those spells are the magical equivalent of the Sniper special rule. Other spells are generally resolved following the rules for "Unusual attacks" on p. 42.

BRB p. 42: "There are several circumstances in Warhammer that call upon you to inflict hits upon an enemy — spells being an obvious example (the Fireball spell causes D6 Strength 4 hits on an enemy unit), as well as things like the Impact Hits made by a charging chariot (covered in the Special Rules chapter). Such hits are resolved using steps 4, 5 and 6 of the rules for shooting attacks."

as well as:

BRB Official Update Version 1.9, p.8:
Q: When hits, other than from shooting and close combat attacks, are inflicted upon a unit how are these hits resolved? (p42)
A: As per the ‘Resolving Unusual Attacks’ box. Note that all hits resolved in this way will count as shooting attacks for working out who is hit.

So, unless specified otherwise, spells will follow the rules for shooting, and you will have to randomise the hits, when targeting a ridden monster. Of course, the rules for spells using templates do specify otherwise, and such spells will hit both the rider and the mount, as per BRB p. 105.