hi @rj09
para sa akin, you need the generic COE from all the companies containing the company letterhead, job title, duration of your employment and whether it was fulltime or parttime work. photocopy mo ung COE and then have the copies marked as "certified true copy" by the notary/law office to pass to ACS as proof of your employment.
then to prove ung job responsibilities mo is pasok nga dun sa job code na pinapa-assess mo, dito mo kailangan nung affidavit/stat declaration from your ex-colleagues. you don't need the company letterhead for this. but the document should contain yung name nung person, their particulars (identifiers like passport, driver's license, etc), how you know each other and then what they're declaring about you (that you worked together and what job responsibilities you were doing).
note that your ex-colleagues should present themselves before the notary/law office at dun lang nila pipirmahan ung document and swear before the lawyer/notary public about the truth of the document.
look up on samples of affidavits or stat declarations para mas may clarity ka about them.