应该总共是some difficult ones like A,B...吧
这当中的ones应当指代前文中提到过的东西,是个名词
比如We met with several problems,and some difficult ones like changing the schedule and raising more funds are really tough to solve.
这句当中ones指代前面的several problems
按这个方法你也可以找到你这一句里ones的含义