You would definitely require the experience letter from your previous employer whether you choose EB2 or EB3. EB3 would be a better choice rather than to stick into RFE and audit later on.

Normally, your previous employer will give you the experience letter, I do have the same situation like yours, not a good relationship, but eventually if you follow up and make two three calls every week and bother them, they will give you the letter at the end of day. But remember, you need the experience letter from Previous employer.