"1/2 as effective against X" Limitation and Player Controlled Allies

[Peaches]

Is there a RAW legal way of having a limitation for making an ability that is only half as effective against a certain category of targets? What about 1/4, or even an enhancement that makes it x2 or x4 more effective? I'm thinking primarily in terms of using it to modify Injury inflicted, and I know I could give all the targets the appropriate advantages and disadvantages to simulate this, but assume this is not desirable.

If all Player Characters have certain kinds of Allies which are Player controlled but not Player Character controlled, should this simply not be paid for in Character Points but be a campaign switch?