Which is the best way to get a lazy yet intelligent associate/agent to get the assignment done on time?

take the help of icentives... financial or non-financial incentives can motivate them to work

What Rubeena said is correct i agree with her words at the same time by providing a gifts which are useful in their daily life like Hoodod jackets,,,.etc,.

Then they will loose their lazy.... and work with intrest and intelligently...

He is a stakeholder. Formulate a strategy to take care of his needs, expectations and aspirations and implement the same accordingly. 

MakeMake him   part of a team ,value his opinion,understand his way of doing things ,make him feel important,explain to him the department and company vision and mission statement,ask him for solutions to problems that He isn't a part of,and most important communicate to him the risk of not completing the work on time on you,the department and company.

Key word make him feel needed and important,trust me it works like magic

First step would be assign the job with a deadline otherwise open time means productivity will be low.

Second step - daily follow ups if its long cycle job if short hourly follow up on update (not in other cases as this person is lazy so he/she wont do it in time)

Thid step - announcing reward for quality work but if its very minimal or not so important work giving a reward or incentive will not be feasible , then above two will be important 

Once there is observation from superiors laziness will surely be tackled otherwise he/she will be afraid during appraisals (yearly /half yearly) as performers will get better appraisals and lazy will get minimal or even will be shown door