I honestly couldn't tell you but I do know that from my experience the medal didn't "unlock" in the way the game usually awards medals until the end of the game although it did show up on my screen as achieved.
If I had to guess i'd say you need to involve a death, as I know I did plenty of X2 and so on well before I actually saw the Combo medal in my unlocks. My strategy was, if you notice where the X2 ect are shown there are about 4 or so slots that light up when you do shoot something or an enemy is hit with something. Example: shoot an enemy with the semi-auto rounds and the gun icon will light up, but if he's on fire first the fire icon (guy next to fire) will be lit. Shoot him then and both will light giving you in my estimate, a combo. Just a suggestion through my 1 day of playing and finishing this.
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