This is probably fairly old news, but for anyone who's having trouble racking up 7,000 points in a single combo for this medal (like I was), the good news is that you actually don't need anywhere near this many points. This may be as a result of the game being patched to decrease the amount of points required, but while I was playing last night in order to get the Mega Combo medal and, subsequently, the Celebrated trophy, I selected my Semi-Automatic rounds and shot a glowing gas tank as a henchman walked past it. The explosion killed him and also detonated a red barrel beside it. I then shot another couple of barrels in close proximity for good measure, and received a score of 1,250 points for the combo - which then unlocked both the medal and the trophy.
For reference, I did this in the CycloConverter B room in Act 2, where you can also obtain the Espionage medal, although there are also numerous other places where this combo could be pulled off. I was also playing on Hard difficulty, as this seems to award more points than Easy or Normal. The Semi-Auto rounds were used so that I wouldn't have to potentially interrupt the combo by charging up Sonic rounds - Semi-Auto rounds can be fired immediately.