you are doing nothing wrong annie15, you probably chose tank as her second talent, which if you read the talent closely the reason she loses health. She gets a damage buff up to 500% if she is tank. If you are not healer the max is 250% since she always drops 2% health till she is at 50%.This is actually done well cause in withered scenario i remembered you couldn't heal withered. Glad that blizz considered healers this time.