The first phase of the fight is easy enough. Keep on the defensive and perform parries to create an opening. From there just push through and press the advantage. This is the simple part of the fight. When Bellec has lost a quarter or so of his health, he will be staggered and on one knee. Approach him to begin a quick time event.
Mash the shown button until Arno throws Bellec through the door. Bellec will return the favor. After that, the dueling pair is back outside.
Wait for Bellec to attack. Get a perfect parry off and then press the advantage. With a decent damage weapon you will floor Bellec quickly and trigger the next cutscene. Given the chance he will use heavy strikes and his own version of the staggering strike. Avoid these attacks as much as possible.
Once again you are in a quick time event. Pay attention to the button on screen and press it. This will quickly get you through the events that start unfolding.
The fight resumes in front of the massive window. It will be getting more difficult here as well. Bellec will be performing feints, attempting to attack more frequently and the timing for blocking and parries get much smaller.
Once again you will encounter a Quick Time Event by approaching Bellec. Mash the button to get through it successful. This will throw the two of them inside the church. The two just keep talking for a while. Bellec and Arno are very much at odds right now.