If you diet and lose lots of weight you will be likely to have some loose skin.
If you water fast, the body tends to adjust the skin to the size of the body.
Water fasting would be a good way to go for preventing the loose skin.
I'm not positive but I think shorter fasts like ten day intervals may be more helpful in preventing the loose skin because it would give the body more time for the skin to adjust in size.
Plus to achieve that much weight loss you may want to plan it out over a period of time to achieve the goal.