Follow Pierre as he starts running down the corridors of the Bastille. He will lead you to a locked gate door that is barring your way forward. Time to start fighting to buy him the time he needs to open the cell down.
You have 2 guards to start. They are nothing too hard to handle. It is very possible to quickly take out both of them with Parries and then floor them. Just a bit after you defeat them, 3 more Guards will make their appearance.
Take a moment to heal as they approach you. This should be a priority if you are at least than half health. After that you need to just focus on the fight. Keep an eye on your health and keep it up. Pierre gets through the door after another little while. When he does, he prompts you to use your Smoke Bombs to break away from the combat or to give you a distinct advantage in combat. Using the Smoke Bombs will stun the enemies and technically disengage you. You can also use this to quickly dispatch the 3 additional guards you came after you during the second round. When you defeat these 3 Guards Pierre will get through the door. Start following him immediately. Drop a smoke bomb here as well to break any line of sight the guards have on you to avoid getting shot.
Head forward to the next room. Pierre will drop a smoke bomb. Just run through the smoke and follow him to the left. He will drop a smoke bomb and stun the enemies here. Ignore them and follow Pierre down the hallway over to the open window.
Head over to the right and go across the posts and rope. Keep up Pierre as he starts climb up to the ramparts. Take the wall up to the top and keep following Pierre. Go to the right and through the open door with him. Run with him through the next few rooms. He will find another locked door. The guards will catch up to you on the other side of the door. If you have missed any of the optional objectives, this is a good time to finish them.
Quickly get into cover behind any of the boxes in the area. You can then use a Smoke Bomb to break their line of sight and stop them for shooting at you. This will count as an evasion and let you get the upper hand in this conflict. Be sure to watch their life bar as well for opportunities to parry their incoming attacks. You can't move forward until Arno and Pierre kill the 5 guards.
After taking care of the guards, continue forward, following Pierre. Head over to the Northwestern corner of the crossing bridge. Head over to the West and free run up the posts. Get to the grated window then start climbing upwards after Pierre. Keep after him as he goes to the left and around the next corner. This will lead to a cutscene where they will both be jumping off the top of the rampart. You will regain control of Arno. Just follow the prompt on screen to perform the Leap of Faith and complete the memory.