I wouldn't consider that phrase slang. To me, it even sounds a bit formal. To excel means to do something so well that you exceed what is typically expected. Some examples I can think of are: "I have to excel at English to impress my new teachers in America." "In order to be noticed by the casting director, I have to excel on stage." "Being an average singer simply isn't good enough. I have to excel if I want to win the competition."

Nick, Yes you are correct. 'Slang'....it doesn't have an informal connotation, so it could be used however and whenever you want. "He/ she excels in sports" "Bill needs to excel in his writing skills" "She excels at public speaking" Hope this was helpful