After speaking with Achilles, quickly leave the manor and go meet with the screaming man.
You will then have to run with him to the river, where his friend is being pulled by the current towards the waterfall.
Optional objective: Do not touch the water while rescuing Terry.
In order to save him, jump between rocks and tree trunks on both shores. Eventually you should reach a branch above the waterfall from where you will have to jump down to rescue the man.
If you want to complete the optional objective, you cannot touch the water. Run as fast as you can so that you can jump straight to the man from the last branch. Note that even if you dive right beside him, you will still get full synchronization.