I'm still too excited over how I was finally able to get this that I have to share, I thought it would elude me forever.
We're at the finale and my team is getting whupped (the only thing about being on a crappy team is the likelihood of getting a shot at this). I was a Spitter and immediately ran into the open to get killed with the hopes of becoming a Hunter or Charger, something with actual incapacitating abilities. What do I become? A stupid Boomer! I began in the building with the bar. The Tanks were busy climbing the bridge to attack the Survivors, when lo and behold a very weak Louis heads down to sacrifice himself. He was inundated with zombies and was just meleeing the crap out of them on his way around the fence. I ran up behind him and started scratching away, praying he would not turn around and shoot me. I don't think he even realized I was behind him! Three swipes and Louis goes down before reaching the generator - up pops Sacrifizzle! Woohoo!
I wonder how many people have gotten this achievement with a Boomer?