"Greater Master of Terror (Ex): Beginning at 4th level, the increased difficulty of all save DCs for all spells you cast with the fear descriptor increases from +1 to +2. In addition, your fear spells are so potent, that they can even affect individuals normally immune to fear, such as a paladin, although they still gain a saving throw to resist the effect. A target whose HD exceed your caster level by 4 or more is immune to your mastery of terror. For instance, if you are a Sorcerer 7/Dread Witch 4 (overall caster level10), a paladin of 14th level or higher is immune to your fear spells."
How I read it, the fear spell affects any creature normally immune to fear (for any reason). I don't see anything limiting the immunity to just a "Fear Immunity" ability (like a paladin). But I see it affecting a creature immune to mind-affecting abilities, since in an indirect way, they are immune to fear.
Also, I read that its the ENTIRE spell affects the creature, not just a fear portion.
But I do agree that there might be a line somewhere, like a fearful empowered fireball affecting a fire immune and fear immune creature, either dealing or not dealing the fire damage part to said creature.