Ok, I'm going to make a deal with anyone who reads this.
I've just finished up 'Alpha vs Omega'. I have a save on the roof of the RF Plaza when the military shows up, with 5 survivors (including Denyce), all armed with Six Shooters. They can easily take out Boykin in ten seconds. I am happy to take anyone through this part if you still need this achievement.
BUT...I expect you to have the courtesy to help me out with something I need to get as well. My Gt is Helios007. Send me a message on Live and we'll sort something out.