For the purposes of applying for CEC, there is no need for them to mention that you were terminated. Give them the CEC checklist so they know what's required. Also, would it matter to CIC that you were terminated IF you in fact did the duties and responsibilities associated with your role? I don't see why or how it matters.
Did you already have one year of work experience by the date of that experience letter?
As jes_ON suggested, apply for your ROE, submit your paystubs(you can submit the ones for the period not covered by the letter you currently have), any other documents you have that indicates you performed those duties. It would be a good idea to write a cover letter to explain why you are not able to submit a current employment letter, etc.
Where you are in litigation with them, they are probably(on advisement of their lawyers) not going to communicate in any way, shape or form with you. If you win your case or reach a settlement, perhaps you can consider asking for this employment letter as part of the deal and also for someone(who is not directly involved in the case) to provide their contact information in case CIC calls to verify your employment details and the person should be required to provide true and factual information and do so in good faith. Since you will need a reference for future job applications, also consider adding this as part of your deal.