One thing, when dealing with a Topnotch is getting up in his face to deal with him. You can also use his "staying in the air all the time" against him by not necessarily having to get him to megablast, but just hitting up farther upwards to knock him out (depending on how high he is of course).
I think folks are mainly still learning the heroes and how to deal with them effectively, he has the highest damage a lot because he is being left alone in the skies above, with no one dealing with him properly, allowing him free reign to do whatever he wants.
Get up in his face some, and try pushing him out by hitting him farther upward, you'll be surprised how well that works. 🙂