Floodgate is one of the easiest levels within the four games.
You can run around almost all the flood, If done on easy/normal difficulty.
You will find a gravity hammer just around the first building. Trust me, the gravity hammer will be your best friend. I got a par time of 4:36 on normal difficulty.
For you guys going for the Devastating achievement, I recommend watching this.
All the credit for the video goes to Nak3d Eli
One of the best speed runners around.