This achievement is for finishing every story mission with ALL optional objectives also met. I wrote guides for them listed in the spoiler below. If you finish a mission with a failed objective, you can always replay the mission from the DNA Tracker from the Pause menu
. If you fail or are going to fail an objective before you advance farther in the mission, you can often restart from last checkpoint from the Pause menu also.
*** Spoiler - click to reveal ***The individual story achievements do not require optional objectives, but I included strategy for 100% on them anyway. And you need 100% objectives in one playthrough of each memory to get the Perfectionist achievement. You can click any of these that you need.
If you need more assistance, then you'll want to go here:
Assassin's Creed III Remastered Walkthrough by
xDaff