If your employer does not provide you with a job duties letter, you can have it signed by one of your colleagues or seniors also.
You can do it on a simple paper only.
- Have your work experience letter from your employer (Which might only include your employment start & finish date, your post on its letterhead)
- Job duties letter (signed by your colleague/senior)
If you need some guidance on format of the same, then please follow these links;
http://www.canadavisa.com/canada-im...tificate-t487385.0.html;msg5959284#msg5959284
http://www.canadavisa.com/canada-im...tificate-t487387.0.html;msg5959302#msg5959302
Hope it helps.