Not sure if this is a bug, a glitch, or by intention but the criteria for There Can Only Be One don't appear to be what it says they are. 

According to the achievement text you have to kill off 5 minor factions. I did this on Buzzsaw - 10 player map with 6 Pirate factions, but never got the achievement.

I tried again on Ashred - 8 player map with 1 of each faction type (Pirates, Aluxian Resurgence, Pranast United, Jiskun Expeditionary Force, Viturak Transport Cabal) and successfully complete it that way.

If this is the intention then it's not a big deal, but the explanatory text probably needs rewriting as it's deceptive.