Give a mob an effect when you look at it.

I want to make an armor set that giev the wearer the following ability: when I look at a mob while I am wearing the full set of armor and press a keybing the mob should get a specific potion effect or be set on fire. Will someone please send me a tutorial with pictures of the code block setup on how to do this. I think I will have to use raytrace blocks, but I don't know how.