The first thing you should do is learn how to aim well and estimate your opponents location properly.
There are a few types of combos, but I am just going to share some.
Straight-strafe combo:
Hit your opponent and move backwards for a bit while he is taking knockback from you so that you are out of reach. Later (after 0.6 second), quickly sprint forward, hit your opponent and move backwards again.
I sometimes get called the hacker for anti-knockback/reach/killaura when I use this strafe.
Circle-strafe combo:
Sprint around your opponent and hit them every 0.6 seconds. This is deadly to your opponent as you will receive very less hits. But you must have an extremely good aim, otherwise, you will look like dancing around the player. I personally prefer the straight-strafe combo because I will get more hits.
I sometimes get called the killaura/aimbot hacker when I use this strafe.
Hope that help! I am planning to create a tutorial for 1.9 PVP soon.