When you escape Maxos temple and get to the Sentinel Island you will have to pick your staff from two possible candidates for each job (skill trainer, alchemist, necromancer and enchanter). Each candidate has different strengths and who you choose is irrelevant. Once you gain control of the Battle Tower the staff relocates from the island and move to the tower. At this point you can visit them by traveling to their floors using the Dragon Stone.
When you talk to them, you will have the chance to ask them how you can support them and enable them to provide better services for you. In return they will ask you to do favors (killing people, collecting stuff, etc). Finish these quests and then go to your living quarters and talk to Sassan. You will then have the option to ask her to upgrade the respective platform of the tower. Once you finished all quests for all of your staff and talked to Sassan afterwards, the achievement unlocks.