The characters in Mortal Kombat 11 are an odd bunch. Erron Black is one of the most powerful characters thanks to his ability to control where your opponents are and his ranged attacks. He also has some truly obnoxious combos thanks to his great mix-ups and spacing too.
Even his basic combos have a surprising amount of strength to them. His basic Into the Badlands combo has a decent amount of damage and also knocks enemies away allowing for further combos. It even works at the beginning and the end of a string of attacks.
On the Shoot is an exceptionally fast combo that can be special cancelled after the 4th hit. It makes it incredibly powerful and versatile and will leave your enemies scratching their heads. It also becomes a Krushing Blow if it is a counter hit.
Dead Man's Hand is another good one as it can be special cancelled and is a great one for corner combos.
If you like drop kicking enemies then use Boot Drop. It hits crouched enemies, has a decent range, and if you connect with it twice in a row it'll become a Krushing Blow.
In terms of special attacks, it is well worth learning Zaterrean Spit. This chucks out a pool of acid that deals damage over time to the other fighter if they are stood on it. This is one of your best tools for controlling the fight and forcing your opponent to either stay away or come close. If you throw someone on it then it deals damage throughout the animation too.
Locked and Loaded is a very deep attack. It makes you walk forward with your rifle out. You can then hit attack to fire up to 8 bullets. If you want to though you can crouch and fire or even hit the enemy with the butt of your rifle. It lets you disguise your attacks and really let you play with their heads.
While he has plenty of other attacks too, these are some of the good ones to start with. You can look up the inputs in the moves list and it is well worth mastering them to get the most out of this gunslinger.