How to deal with a difficult employee at work to complete the task?

Depends on what you mean by difficult, you need to know why the employee behaves this way.

If he\\she complains a lot about how things run in the department then delegate more authority to them and give them a dead line to produce the needed results with a penalty if they don't deliver.

If he\\she just refuses to work then explain to him\\her how this behavior is going to effect their performance review and delay their promotion.

If he\\she don't understand the job roles and responsibilities then you need to couch and train the employee.

So it depends on what you mean by difficult, First dig deeper and know why they are difficult, Then use the appropriate action you see fit for the situation.