Against the Citadel Ascendant, ignore their and allied fleets and attack the starbases. Spire ships work best here.
Against the psuedo-devourer: no clue. My first attempt ended in utter disaster.
Going to echo this for the Citadel Ascendant.
Go straight for the enemy starbase with a spire ship. Pull back after you pop one starbase so your ability points fully recharge your shield ability, then go full attack for the second one. Playing defense simply cannot work, but playing fully offensive can work. Load up on polarizers and plasma cannons if you have them.
I actually got defense to work on the Citadel Ascendant--it was the first time I tried it, so I had no idea what to do. Basically, I used the Neinzul ship, focused on killing the Colony Ships that went for allied starbases, and used the repair ability whenever the field looked somewhat clear on that side. Note that worked != ideal. The scenario took me a very long time to do that way, required more attention than most scenarios, and probably raised my blood pressure a few points. I did keep all of the Allied starbases alive, though. I don't know if Nebula difficulty is affected by AI difficulty; I was on 7 at the time.
Again, I do not recommend that approach. Just saying it has worked before.