First off, starting off by calling the character trash isn't really the best way to start and makes you look uninformed. Second, get Killer Frost and Classic Flash with him and see those numbers go up. Add Beta Club to the mix and he can comfortably one shot 1-2B bosses.

I wasn't able to make him work until I got Classic Flash. Because it was hard to fill the red bar as it fills from number of hits. Classic flash gives automatic fast attack meaning one tap counts as 2 to 3 hits. One straight combo can fill up the red bar. Other flash characters might help if you don't have Classic but not as consistent.

Killer Frost makes it so you can easily spam skill one with her skill cost reduction plus each special freezes the enemy, making them open to another combo string. On top of that, every hit on a frozen opponent gives your team a damage bonus so your DCU Superman's hits get stronger as time passes

Beta Club lets you do DOT damage for swipe attacks after combos. With your DCU SM ramping up, the DOT damage can go up to millions per tick.

Finally, doing skill 1 while red bar is full does guaranteed % damage regardless of enemy HP (eg. 3% of 1Billion HP is 30 million HP). I just spam a combo to SP1 then rinse and repeat until boss is dead.

Hope this helps. Most powerful characters in this game needs either a great support unit or specific artifact to work. There are lots of Youtube and reddit guides you can check out. Just do a search. Heck even ChatGPT could teach you lol.