Put her full weight on her.

Grabbing both of Rangiku's arms and pushing them to either side of her body so not to push her off, Tsunade lays back down on Rangiku, this time with her legs ontop of Rangikus as to press her full weight on Rangiku's front.

"Eugh, get off!" Rangiku once again weakly struggled, her breathing becoming laboured from the heavier woman just laying on her. Tsunade continued smirking, her face only seperated inches from Rangiku, the huge chests of either lady pushing the other away. "Well this is a cosy new mattress, springy too" Mocked Tsunade, doing small bounces ontop of Rangiku, each having little more effect on Rangiku.

"Get. OFF!" screamed Rangiku once more. "How embarrasing would it be if you got beaten by me just laying on you" giggled Tsunade. "Not very embarrasing considering what a lump you are!"
The second fat insult hit the same nerve the first one did, but Tsunade simply tried to shrug it off. "Aww struck a nerve did I? A fat nerve?" Mocked Rangiku once more, with Tsunade gritting her teeth in response.
"The great fat cow Tsunade, squashed me with her huge fat udders!" Rangiku tried a small laugh, her breathing however still laboured.
BOOM.
A loud shriek stung the air, as Tsunade's fist lay ontop of Rangiku's left shoulder.
"MY ARM, YOU BROKE IT", screamed Rangiku, Tsunade's full weight no longer still on her.
Tsunade grabbed Rangiku's hair and pulled her closer. "I'm gonna put you through hell for those comments."