In everyone's life,not only one will meet various difficulties that he can't solve【去掉 it】.but also these difficulties will puzzle 【one换成him,避免重复】 constantly.
也可以说成：
In everyone's life,one will not only meet various difficulties that he can't solve,but also will be puzzled by these diffculties constantly.
这样 not only but also 连接的句子主语都是one,更舒服些.