someone could do something with one arm/hand tied behind his/her back idiom informal
易如反掌（形容可以轻而易举做到某事）
If someone could do something with one arm/hand tied behind their back, they can do it very easily.
Her part wasn't very demanding - she could have played it with one hand tied behind her back. 她在戏里的角色要求并不是很高，她能轻而易举地完成。