I can't seem to find a definitive answer to this question: If an incorporeal creature casts a spell at a corporeal target, does the spell do only half damage, or does it do full damage? Under the description of Incorporeal, it says that an incorporeal creature struck with a non-force based damaging spell only takes half damage. Under the Sorcerer bloodline, at 15th level they get Incorporeal Form as a spell-like ability, and the description states that the sorcerer's spells do only half damage to corporeal targets. However, in 3.5, it states that spells cast by an incorporeal creature affect corporeal creatures normally. I realize that the last was taken from 3.5, but it is the only reference I could find that stated the effects of spells cast by incorporeal creatures, rather than a PC under the effect of a spell (or at least a spell-like ability).
I've been wondering the same thing. I'm also curious if spells with no physical manifestation (like enchantments) can ignore all this nonsense.
I can't give you a direct reference, but I just finished playing in a pathfinder module where there was a ghost wizard. He only prepared force effect spells (like magic missile, shield, and mage armor) and summons. I.e. he avoided all the spells we are wondering about.
By this I can infer that either the other spells were reduced in effectiveness, or the writer of the module didn't know either. :)
The only good news I can offer is that it looks like conjuration(summoning) spells are good to go.
Never worry about what 3.5 says.
Incorporeal (Ex)
An incorporeal creature has no physical body. It can be harmed only by other incorporeal creatures, magic weapons or creatures that strike as magic weapons, and spells, spell-like abilities, or supernatural abilities. It is immune to all nonmagical attack forms. Even when hit by spells or magic weapons, it takes only half damage from a corporeal source (except for channel energy). Although it is not a magical attack, holy water can affect incorporeal undead. Corporeal spells and effects that do not cause damage only have a 50% chance of affecting an incorporeal creature. Force spells and effects, such as from a magic missile, affect an incorporeal creature normally.
An incorporeal creature has no natural armor bonus but has a deflection bonus equal to its Charisma bonus (minimum +1, even if the creature’s Charisma score does not normally provide a bonus).
An incorporeal creature can enter or pass through solid objects, but must remain adjacent to the object’s exterior, and so cannot pass entirely through an object whose space is larger than its own. It can sense the presence of creatures or objects within a square adjacent to its current location, but enemies have total concealment (50% miss chance) from an incorporeal creature that is inside an object. In order to see beyond the object it is in and attack normally, the incorporeal creature must emerge. An incorporeal creature inside an object has total cover, but when it attacks a creature outside the object it only has cover, so a creature outside with a readied action could strike at it as it attacks. An incorporeal creature cannot pass through a force effect.
An incorporeal creature’s attacks pass through (ignore) natural armor, armor, and shields, although deflection bonuses and force effects (such as mage armor) work normally against it. Incorporeal creatures pass through and operate in water as easily as they do in air. Incorporeal creatures cannot fall or take falling damage. Incorporeal creatures cannot make trip or grapple attacks, nor can they be tripped or grappled. In fact, they cannot take any physical action that would move or manipulate an opponent or its equipment, nor are they subject to such actions. Incorporeal creatures have no weight and do not set off traps that are triggered by weight.
An incorporeal creature moves silently and cannot be heard with Perception checks if it doesn’t wish to be. It has no Strength score, so its Dexterity modifier applies to its melee attacks, ranged attacks, and CMB. Nonvisual senses, such as scent and blindsight, are either ineffective or only partly effective with regard to incorporeal creatures. Incorporeal creatures have an innate sense of direction and can move at full speed even when they cannot see.
Nothing in there about the incorporeal creature having less effect with its abilities.