Like anything else in the game, it depends on the special effect, and what you want the pain to do.
If the goal is to knock someone out some form of Stun only attack will work. This could be an NND or other exotic attack or just a plain Stun only attack.
If you want the pain to cause other effects, you need to build those using other powers. One way would be to use a limited form of mind control. A single command drop the item you are holding because it is causing you great pain would simulate this. Another way would be a change environment requiring an ego roll to continue your action. Like I said it depends on what effects the pain causes.
This question is kind of like asking, how would you build an electrical attack. There are many right answers to both of these questions.