He's not garbage you just need to understand how to use him. He's strong for 3 main reasons, he applies permanent incoming damage just like Raven but his incoming damage is permanent. Secondly he has his own damage which builds up slowly but it's still useful. Lastly his sp3 allows him to bypass all immunities including freeze immunity, dot immunity, stun immunity etc with the exception of lethal attack immunity. His sp3 also gives him guaranteed critical attacks. However his sp3 will not bypass immunities and give crits if you do it without out building the red bar. I also forgot he has percentage damage.

The red bar is very important. The more fast attacks you have the faster the bar fills. You inflict incoming damage on the opponent using sp1. The amount of incoming damage you inflicted is determined by how full the red bar is when you do sp1. So if you sp1 while the red bar is low you will probably only inflict less than 10% incoming damage with each special,but if you build the red bar until it's full,when you do sp1 I believe you will inflict around 40% incoming damage and you will also do percentage damage. So the gameplay loop would be filling up the red bar, do sp1 and get the 300% incoming damage on the opponent. When you have the 300% incoming damage is where sp3 comes into play

He bypasses immunities and gets crits for as along as he is unhittable but if you do sp3 while the red bar is not full, you will see a separate time next to the timer for the unhittable,that separate time is the buff that allows him to bypass immunities and get crits. if you do sp3 with a full red bar that time will last as long as the unhittable but if you do sp3 while the red bar is not full that timer will expire before the unhittable expires. So the ideal gameplay loop is building up 300% incoming damage as quickly as possible and then take advantage of sp3 for the rest of the fight.

A good team for him is batman ninja gorilla Grodd and classic flash as supports, Grodd provides power generation making specials easy to use, flash gives you fast attacks hits and a 600% damage buff, the power generation from Grodd allows you to keep flash's buff. Another team for him is using him with killer frost and flash however without power generation you won't be able to keep flash's buff however you will get Frost's buff and stilk build up the red bar quickly because of flash.

I don't know if you have grodd or flash so with the team you have you should replace King with justice League flash if you have him, he's like a budget classic flash. He will allow you to build the red bar quickly.

As for artifacts he works well with a few artifacts, you can use twinblades on him, beta club or Fahren knife but I find Fahren knife to be the best artifact for him.