Injustice 2 will boost your power, if you purchase it the right way

Now that I finished the story I’m having a tough ie. way slower time getting loot or leveling up characters. What is the fastest way to do this for someone who sucks? I thought about trying some online ranked matches thinking maybe that’s the best way, but I’m not sure it would be any better than losing in Multiverse and ending up 0-30 (I don’t really care how low I’m ranked). I have some neat loot pieces that are level 20 for 4 characters but that seems like a very long way off.