对许多人来说，糖也许早就是久攻不下的顽疾了。

Sugar for many is something they may have been battling with for a long time, but the past is the past.

智能钻杆作为智能钻井的核心技术，一直是国内研究人员久攻不下的技术难题。

As the core technology of smart drilling, intelligent drill pipe has long been a technical problem which can not be solved by domestic research staff.

我们的计划就是保持球门不失，并在对手久攻不下开始感到沮丧时发力攻击。

It was our plan to keep a clean sheet and start attacking when the opposition was getting frustrated.