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No you do not to just reference letter from your employer. Stating your role
And the no of hours you worked (very important) and your remuneration
 
For applying as a federal skilled worker , do we need a job offer in skill level A, B ,0 from a Canada

No - you don't need a job offer. You can apply without one. Although having a job offer gives you more points.
 
Job offer from Canada other than skill level A,B,0 is acceptable or not while applying as federal skilled worker?

Not acceptable. Only a NOC A, B or 0 job offer is accepted under FSW. It will need to be supported by an LMIA.
 
I have earned a two year Canadian diploma other than the education from my home country and I have a cousin living as a permanent resident in Canada. Is there any advantages for these factors while applying as a Federal Skilled Worker?