after trying many methods listed here to kill Dalton in less that 40 seconds and none of them working, I said fuck it and we both just grapped our fully upgraded AK-47s and he fell before getting to the corner. prob 10 secs. Then he went around the right, and I went around the left and we finished unloading on him from both sides, still with our AKs. worked like a charm.
The problem with the rocket launcher is that the only one with enough ammo takes about 15 minutes to reload the damn thing. And the problem with the snipers is that too often, he would duck down for too long, wasting valuable seconds and there are lots of obstacle in your way. So it's really hard to get an open shot.
I'm not saying those methods don't work. I'm sure they do. But once we switched to the "AK only" method we got it on the first try.