Army of Two

Ok this one is very easy with another person in co-op and it has to be on the last level with the alternative miami ending. If you have a friend just go through the normal routine of taking down enemies and when you get a chance knocking Dalton until he gets to the group floor. Once you have him on the ground floor have your friend take on aggro while you get some shots on Dalton. Wait until he has barely any health then stop shooting him.
While your friend still has aggro crouch all other way around to Dalton taking out any enemies that stand in your way. Make sure your friend keeps aggro but also stays alive. If he dies the aggro goes to you and you are pretty much screwed. Once you get close enough to Dalton take out your pimped .44 MODEL 29 handgun and rush up behind Dalton. It is very important that you don't run up too close to him. It is also very important that your friend keeps up the aggro if he doesn't you don't have a good chance at killing Dalton. Once you are behind him unload with the handgun making sure that you have a good shot at his head and not the neck or back. A few shots in the back of the head should do it and give you the achievements. Good Luck
-BTY

For those trying this with the AI partner only, here is what was successful for me (2nd try...and I'm not a pro by any means.) I selected the AK-47, pimped out, upgraded to max damage and max aggro. When scene starts, use AK-47 to bring Dalton down off the catwalk (set my AI partner to full aggro to assist). After the second cutscene, I took my AI partner down to cover, and gave him the AK-47, then put him back on full aggro. Meanwhile, I broke to the left of the circular room and made my way around to Dalton, taking my time, killing randoms in between. My AI was barely getting in shots on Dalton, but had high aggro, so I unloaded on Dalton with the assault rifle given to me by the AI partner until Dalton's health was almost gone. I allowed my partner to keep/build enough aggro to keep Dalton's attention then shut off the AI partner so he wouldn't accidentally kill him. I got near enough to Dalton for a headshot with the pistol...hit him with two and he died. Similar to the above solution I know, but slightly more specific for the solo player.