If there is no way you can get a letter issued by the HR/manager/supervisor on the letter head, then you have no other way, but to look for alternatives to prove and convince the Case Officer the genuineness of your situation. So, if you can arrange as many documents as possible, stronger your case becomes. But that is not a guarantee for it to be accepted, for some people it was accepted while for some it was not. I would suggest the following.
1. Get copies all the documents which evidences your employment (employment contract, offer letter, pay stubs, tax papers, appraisal letters, promotion letters, bank statement to show salary credits etc)
2. Since you said that your manager can issue you a letter (I am not sure whether you meant on the letter head), get it on white paper, get it notarized, along with a copy of visiting card and employee id card copy (also if you have any communication where your HR refuses to provide you the letter, please keep a copy of the same as well)
3. Prepare an LOE explaining your situation and submit your documents.