Sounds to me like you're making basic errors somewhere, because you seem to have a lot of success up to a point. You might consider looking at the wiki so as to get a better grasp on the numbers behind health, damage and dodging.
Also, the Wall is easier than you think. The enemies are easy to beat when they're trickling around a corner in ones and twos. Don't blow a hole in the middle of the wall. That missile launcher will be your best friend if you experiment with it a little.