The problem with a dent is you stretch the steel. When you pound it back you're likely stretching it more. What you need is a good hammer and dolly man who has "shrinking hammers" and the right tools. A shrinking hammer (not sure if that's the right word) has a waffle face similar to a framing hammer. It creates a waffle like texture as it takes the metal which is too big in 2 dimensions and tries to bring it to the right size by adding depth.

Try beating it with a rubber or leather mallet. Maybe a dead blow. Maybe pry on a 2x4 and try to push it?