Currently, I have Dark Sim as a separate button.

I attack priority target first to force them to use a big heal, CC, or immunity.

Right before I think they are going to use something, I cast dark sim on them.

Once I got something, I just spam it back at my priority target.

I tend to use it as more of a psychological weapon. When you are killing someone and you place Dark Sim on them, they second guess themselves. Sometimes they will delay stop doing anything until its off.

Of course these are the bad players...