Army of Two

This is a DLC achievement and you must beat the mission named "Miami Alternate Ending" (Mission #8) on Professional mode with either a co-op partner or AI partner.

You have to remember in Professional mode your enemies will hit harder, take more damage before they go down and your partner will have less time to revive you then in other modes.

I HIGHLY recommend playing with a second person since it makes it easier however this achievement is still doable playing with a AI partner.

- To take out Dalton the easiest I used stingers. Best place is to stand is under the circle walkway he uses. Wait until you got a clear-line-of-sight before shooting.

- The enemies will never stop spawning.

- When he falls down from the walkway take cover all the way in the back of the room. Use the GPS to track him. Toss a few 'nades at him and try your best to hit him from where you stand.

- Take your time. Take cover as much as possible and conserve ammo. You more distance you have the better.

- Work with your partner (even if he is AI). Cover him when he/she needs it. Try not to get into areas where your taking on allot of fire, you may need to do some reviving and heavy fire makes that nearly impossible.

I had to do this on my own because of the servers going down. Don't know if it was a one time thing or if it will always work this way. I started with Grenade Launcher and Stinger. As soon as the battle starts, walk back inside the circular vault you came in from and wait for Dalton to be on your line of sight. You can land one stinger and a couple of grenade launchers every time he appears in front of you. No one will come in and only Dalton would be throwing hand grenades inside, which you can easily avoid. Set your AI to BLUE HOLD. He will always revive you hassle free if you die as you are both inside. Keep using the stinger and grenades until Dalton falls down. Once he does, you will spawn outside again. Turn left and head back into the same circular vault you were before and have your AI follow you with BLUE REGROUP. If you can take out the RPG guy who will be running upstairs to the left of where you spawn, kudos to you. Do that with one grenade launch hit to the wall near him. Once inside I walked all the way back and (this is the part that I don't know if it will always occur), Dalton would set himself right in front of the vault from across the room. I then traded my stinger for the AI's sniper rifle (I think I still had one shot left as I don't think the AI will take an empty weapon). I used an inside wall (right) and the side of the circular opening (left) to narrow my line of sight and I had Dalton and 1 more SSC soldier right in front of me. Dalton would do blind fire and then rise up and show his head. I then landed one headshot every time he did that. I tried not to waste bullets on the SSC soldiers, but if they get too pesky, blow the away. The beauty of this is you get a special weapon spawn right outside the door, so when you run out of bullets run outside and head back in quickly. You will get infinite tries until you get Dalton, as no one will come inside to get you. Remember to leave the AI in BLUE HOLD so you can hug all the action and he doesn't get killed.
As I said, this happened to me once and I took advantage of it and finished it right then and there. I am pretty sure this might work every time as Dalton usually sets himself in front of you from across the room regardless of where you are, and I don't think the weapon spawn was a glitch either. Hope this helps other guys out there.