You must kill Dalton with a pimped .44 model 29 handgun. First equip that specific handgun and make sure the appearence is pimped. Then start a co-op game on Mission 8 on recruit. Once you get to Dalton try to get him down to about 2 bars of health, maybe less, once done get one partner to draw enough aggro to go into 2X damage mode. Make sure you are close to Dalton once in stealth mode, once in stealth mode run over to Dalton and shoot him repeatedly in the head with the pistol. When I did this I shot him about 5 times and he didn't die although once stealth mode ended I shot him once more and I got it. So I don't think you can get it in stealth mode but once stealth mode is over he should have like no health so you can just shoot him in the head. The trick here is to make sure the final blow that kills him is a headshot with the pistol. So there are a bunch of various methods to do this. Just do what works for you.