No one plays campaign.
Kidding. I had that mission glitch out on me before, but it was in a different spot. I just killed myself, restarted from the last checkpoint, and was able to continue. This is right before the cocaine lab, right? You indeed have to kill all of the enemies to continue, and it sounds like one of them is glitching and won't let you kill him. Not much I can do but suggest you try it again, and if it glitches again, restart from the last checkpoint. I've played through each of the campaign levels numerous times and I haven't seen this glitch.
Oh yeah. In the front-right of this area is a bell tower that you can climb (ladder is in the inside). Head to the top (you'll find a Dragunov) and see if you're missing someone on top of the building in front of you.
I had replayed the entire mission a couple times, couldn't find anyone anywhere, including where the intel is in the belltower. This morning I watched a youtube video and found out you pretty much have to push ahead and stay up front, not hang back and cherry pick or you get this glitch. It seems to be fairly repeatable based an a few playthroughs for my play-style anyway. I don't think I've really missed anyone as it does say "that looks like the last of them" but some trigger just isn't happening unless you push hard on that one.
edit: p.s. I was able to get through on my 3rd try by staying way up ahead and making sure I was in the square when the last guy was wiped out.