I had the same operation a little while ago, but never got progress. Any idea whether it has to be a certain situation? You in the smoke and the enemy outside of it, or you outside and the enemy inside? (seems strange to do it that way, though) I would figure having both players inside would be a sure credit though...
OP, maybe change your thread title to reference the specific info you're looking for (how to progress the operation), some may misinterpret what the thread is discussing.
There may be a glitch that needs to be fixed. If you're trying to kill opponents inside your smoke on TDM, forget it.
In Dom, drop a C4 on the flag. I mean right there on it touching the very center of the flag. Then drop your smoke right there with it. May take a little practice to get the smoke nade to bounce just a tad. Then ... OMG ... camp and wait for someone to try capping the flag. Double X and there you go.