[Grammar] This is to certify that the above-mentioned person has been the top student

[drangiana]

This is to certify that the above-mentioned person has been the top student in the last 2 years of his bachelor in the Department of Mechanical Engineering and also has been the assistant lecturer of “Strength of Materials” in the Mechanical Laboratory at this university from September 2012 to 2014.
This is to certify that the above-mentioned person has been the distinguished lecturer of “Know about Research” and “Biomimetics” at this institution from September 2016 to the present.

Is it grammatically correct? Is the choice of words as it should be?

 

[emsr2d2]

This is to certify that the above-mentioned person [STRIKE]has been[/STRIKE] was the top student in the last 2 years of his bachelor's degree in the Department of Mechanical Engineering and was also [STRIKE]has been[/STRIKE] the assistant lecturer of “Strength of Materials” in the Mechanical Laboratory at this university from September 2012 to 2014.
[STRIKE]This is to certify that the above-mentioned person[/STRIKE] He has been the distinguished lecturer of “Know about Research” and “Biomimetics” at this institution [STRIKE]from[/STRIKE] since September 2016. [STRIKE]to the present.[/STRIKE]

Is it grammatically correct? Is the choice of words as it should be?

If he was the only assistant lecturer, you can use "the" or no article at all. However, if there was more than one, use "a".
I wouldn't use "the distinguished lecturer" as there's a danger that it will sound like a job title. You could say "He has also distinguished himself as the/a lecturer of ...".

 

[tedmc]

I suppose that is meant to be a testimonial for a student. What is the purpose of the testimonial? A testimonial would usually have a title. My suggestion of how it is to be written:

To Whom It May Concern

This is to certify that xxxx was the top student in the 3rd and 4th year of bachelor's degree in Mechanical Engineering. He held the position of Assistant Lecturer(Strength of Materials) in the Mechanical Laboratory at the university from September to ??? (month) 2014. During his tenure, xxx received the award of "distinguished lecturer of ..... for the period ....

I think your description is too brief leaving many gaps in between and questions unanswered.
How did the student do in his overall degree? Did the student take up the lecturer position after he completed his degree?
I would have thought that an Assistant Lecturer's work involves more than just being in a mechanical laboratory. Who awarded the "distinguished lecturer" title? Was the title just for a particular topic instead of the full job scope?