This achievement seems fairly straightforward, but keep in mind this is the level cap. In order to reach it, you must acquire every skill in the skill tree. I unlocked this between sequences 5 and 6 (although I did WWI as well). Here are some tips:
1) Do side missions. Not only are they great as far as variety and cast of characters go (if you're a history nerd like me at least) but they give you a lot of XP, which in turn gives you ability points, which allow you to level up (remember, XP isn't enough, you have to SPEND the skill points you acquire in order to level up)
2) Liberate districts. You'd obviously be checking these off if you were going for a complete playthrough but if you're not, free up those districts! They give you a lot of XP! (Specifically the gang wars)
Other than that I don't have much. Play the game and enjoy (imo) one of the better "pre-RPG" Assassin's Creed games!