@ Raj31
We have already seen couple of instances where CIC has accepted "duties" on plain A4 paper (assuming it can't be obtained on letter head) by the supervisor (in worst case from a senior/colleague). Include ur and ur senior's business card (for verification purposes) and try to get hold of some official JD, even its pretty vague, but will support the letter from ur senior. It has proven efficacy.
Another thing u can do is, make HR confess ur duties e.g (a very crude example but surely u can find a genius way to it) send an email to HR having "self declared" duties and ask them " this is what i do and its matches with my attached JD, y dont u put in a letter" and make them respond as "although we agree that u do these things but sorry its our policy not to mention" and then simply send copy of that email to CIC.
Btw from ur post, it seems that u have been asking a skills certificate, u dont need ur senior's/HR recomendation abt ur skills, but a simple description of WHAT U DO, NOT HOW GOOD U R AT IT. No HR would like to give a certificate abt "capabilities" of good performer, as he can use it to obtain better offer.
Self deceleration remains the last option, it has worked in couple of cases but still exact situation in which it worked is dubious.
It will be great, if HR adds the disclaimer otherwise u can provide ur communication (chain of emails).