How should I file if i'm working for a foreign mission in Canada as local staff? Foreign missions are no required to make tax deductions or issue T4's for local staff.

You would want to check with the CRA on this issue. You might not even need to file a Canadian return depending on you residency status. https://www.canada.ca/en/revenue-agency/corporate/contact-information/international-tax-non-resident-enquiries.html

 CRA: Determining your residency status https://www.canada.ca/en/revenue-agency/services/tax/international-non-residents/information-been-mo...