lol reading this made it sound like you were approaching this fight like some kind of Video Game Boss fight. "Flip the fat guy over and attack his weak spot for massive damage" this is what I got from your post.
I remember fighting a few fat guys, good times. I'm about 5'11" 190 lbs, this fat guy I fought was about 6'3" 280+lbs,. I don't know what I did to him (I was swinging at him, so he was backing up), but he tripped in a sort of funny way landed on his own shin and broke his own leg. It was the fastest most decisive fight I was ever in. The guy kicked his own ass.