Some of the Full Synch requirements in this game are pretty painful.
Here, I'm going to include descriptions and possibly videos for how to easily clear the Full Synchs that I personally had the most trouble with/had to retry the most times.
I'll avoid spoilers as much as I can, but just to be safe, you might not want to read this until you get to the actual mission and see what happens.
River Rescue
Full Synch: Save Terry without touching the water.
Spoiler: Highlight to view
The wording for this one is incredibly misleading which is why I failed. Instead of going for the path the camera shows you (to the right), start running along the left path. There are two Wolves, so when you get close to them, start mashing the Jump button to keep jumping forward. When you keep jumping, Wolves cannot pull you into a Quick Time Event because you're in mid-air. Alternate jumping forward and sprinting forward along the visible path until you feel you've lost the wolves' attention.
Stay on the path, otherwise you'll be slowed down by deep snow. Near the end of the path, there is another wolf. You can either choose to bypass him with the jumping method, or (more likely) allow the QTE to happen and quickly kill it that way. Climb onto the snowy cliff in front of you and head to the right, where it overlooks the water. By this point, the log carrying Terry should be below you. Leap of Faith INTO THE WATER. This is where the wording is misleading; You have to jump into the water, but if you do it right, the mission ENDS before you touch the water. You just have to make sure to jump towards the log.
Public Execution
Full Synch: Bodyguards must survive. Kill 2 Militia.
Spoiler: Highlight to view
When the game gives you control of Connor, sprint through the crowd to the very front where your Target is attacking your Ally. Hit the target FROM THE FRONT, not from the back otherwise you'll one hit kill him and the mission will end. This will aggro your Target so that he starts attacking you instead of your Ally and his Bodyguards. Once your target is attacking you, counter him whenever he attacks but do not use a Kill Counter or a Tool Counter. Take this time, ignoring your Target to kill two guards in the area. Once you've killed the two guards, kill your target and you're done.
A Little On The Side
Full Synch: Do not tackle or shove anyone.
Spoiler: Highlight to view
This Full Synch is absolutely bonkers. You're not allowed to shove or tackle any civilians, which you automatically do in this game when you Sprint into them. In the meantime, your Target is fleeing and tossing money everywhere, which puts more Civilians in your way. At the start of the mission, jump towards your Target and keep chasing him. Try not to shove anyone along the way, until you get close enough to see the Animus glow on him. When you do, Rope Dart him multiple times until you get close enough to Tackle him to the ground.
