The Mentor Achievement in Assassin's Creed: Revelations
Have seven trainees reach the rank of Master Assassin.
How to unlock The Mentor
This task is a working task which involves a lot of time spent going back to it. Essentially you take over dens which give you the ability to recruit assassins. The more dens you take, the more recruits you get. You send those assassins out on missions and rank them to 10. Do this by going to any den or near the bomb icons on the map. You can access missions that you may send the assassins on to gain experience. Read the missions details and make sure you have a high percentage to complete the task. You may have to send a few assassins to complete 1 task. Just make sure the experience is worth it. If you stick to trying to level up a whole level each mission you will have an easier time. Once the assassins get to level 10, you do a special mission for them after assigning them to guard a den. This is an indicator on the map and not the same way you were ranking them up. After that mission they may be ranked to 15 which is Master Assassin, but you must do 1 final mission in order to get them to have a symbol instead of a number. You will eventually have all top ranks by the end of the game (the rest after 7 only hit level 10).