Myrmidon with Prometheus S/Kayser and 3-3-5 Trebuchets (...) Played on Very Easy.
In case anyone else is having trouble with the mission (like I did), this does the trick. Fighters and bombers are very easy in very easy difficulty mode (duh) so the only real threat are the capital ships. You need 1 trebuchet to take out the Cain's front beam cannon and 2 for each cannon on the Lilith, that's 5 trebuchets. Then you need 4 more trebuchets to kill the Ravana's main guns (2 each) and the two last to kill the other two cannons that are facing the Temeraire. That adds to exactly 11.
I'm not sure if turrets have more hit points on higher difficulty modes, but I had a really hard time to replicate this tactic in medium, which is the difficulty I normally play on.
Anyway, I'm thankful to the people that contributed here. Before being pointed to this thread I kept failing horribly at this mission.