Ok, at the 5th or 6th segment ( right after the bit where you have to fire the canon three times ) Harry will have to move right through several screens of cannonball fire. They look like gray clouds of smoke.
Helpfully enough, he will approach right up to each little segment and stop. You can then observe the sequence of explosions and proceed when you're ready. They are simple enough to dodge. If you get hit just quit the game and you can continue from the beginning of this area.
Also - When you get past all the explosions there will be a stairway. Follow it down and to the left to grab the flower before exiting to the right.