This Trophy is more about time than skill, firstly you'll need 7 recruits, the first one is given via the story, you don't need any specific ones to fill out the last 6, if you want to get the ones you just need to save or the mission ones thats up to you.
once you have a single assassin, you need to have them leveled up to rank 10, the quickest and easiest way is VIA Mediterranean defense, when they've reached this rank you must complete the first Master assassin mission, this will vary depending on which Assassin den the recruit has been assigned to.
once the mission has been completed (100% synchronization is NOT required) you'll then have to level the Assassin up to rank 15 again the Mediterranean defense is the best way to do this, (plus its a quick way of getting armchair general at the same time), upon rank 15 the second Master Assassin mission needs to be completed the recruit will be declared an Assassin, the den will no longer be attack-able or loose-able once claimed, rinse and repeat a further 5 times if done 1 Assassin at a time, and the trophy should unlock.