Unlike other races, Skaven can voluntarily target ranged attacks (missile fire and spells) at enemy units engaged in close combat with friendly units. First check that the target enemy unit is in sight of the unit targeting it (if the weapon/spell requires a clear line of sight) and then measure the range to the target (not to the Skaven unit engaged with the target). Roll to hit the enemy as normal, then roll a dice for each hit caused. On a result of 1-3 the hit is resolved against one randomly determined unit among those in close combat with the target, on a result of 4-6 the hit is resolved against the intended target.
Weapons and spells that make use of a template must be targeted at the enemy as noted above, but the player is allowed to position the template so that it might hit friendly models as well as enemies. If the template ends up touching models from both sides or enemy models in base contact with friendly models, follow the rules under Shooting & Close Combat.