Free roam combat is really basic, the first part to it is to lock on by pressing L1 (PS3/PS4) or LB (Xbox 360/One) or RMB (PC).
You can dodge by pressing Square (PS3/PS4) or X (Xbox 360/One) or Space (PC). Extra tip for dodging: you can dodge any attacks so when your target is making a chain of attacks, you can dodge all attacks, also you can counter by pressing X (Xbox 360/One) or O (PS3/PS4).
While locking on, you are able to switch targets by using the joystick to target on other enemies (that is if you're against multiple targets).
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This move that I'm gonna explain is pretty much the best move I found out because it cannot be dodged. So to do this all you need to do is press the run button (while fighting) and press the punch button (while holding the joystick to the direction you are facing).
Then press the run button again (don't hold), then press the punch button.
This particular move is basically a rush attack and with it you will win almost every fist fight.
PS: When fist fighting, when you are surrounded by enemies, you can punch from different directions by moving the joystick in the direction of the enemy, or by using the WASD keys.
Thanks Mr Wong