My Hand Legionnaires faction is maximum.
To raise this faction, you can kill lots of mobs in Katta Castellum. Not just the few listed, but the entire Centurion population in the Lcea House seem to have all the same faction hits. I did not kill every one, but every one I did kill had the same. Once this is max, you can repair your Katta faction by going to kill the mobs inside where the Arx Key deposits you. After that, since now your Seru factions are ruined, you can go to Letalis and kill tons of thought leeches. All maxxed out again.