Experience certificate from privious company for GC

My Comp is applying for my GC, I am in the organization for more than 2 years now and about 5 years in US.

For the GC application my comp needs the experience certificate from my older organization which might be difficult to get considering it has to be on there letter head and with my roles and duties.

I wanted to know if there are any alternatives to that experience letter and how important is it for GC. I was with my older organization for more than 7 years.