If you want to do it legit, fast and easy, here's how to do it:
I just did mine with a friend in co-op. He being level 11, me being level 22.
We did the quest ''The Bandit Armada''.
They won't attack you on your own island, so you can gather resources and level up through your ages, and then build the dock to build Triremes/Fireships. I had my fireships upgraded to champions, trireme's nothing special. My friend also didn't had any upgrades for them.
Here comes the nice part. you need to destroy the bandit fortress, the trireme champion ship and 10 normal trireme's. Although youre playing on Elite, the ships and building are NOT elite. So both build a lot of ships, and then attack the normal trireme's and the champion trireme. Then go to the upper center of the map to see the bandit fortress. There will be Elite Toxotes Archers in front of it, take them out first,this goes pretty fast. Then get all your ships on the Bandit Fortress. Ours went down in less then 30 seconds.
All by all, this isn't so hard, but I recommend you to be at least level 10-15 before trying (having Age 3), and set your tech tree to Triremes and Fireships and upgrades.