I got this when reached level 34 but was green for me, so i suppose you can get it around level 30. Its a quest in Argos, repeatable and doable in coop. Dont know if you start on the same map always, i tried it once and the islands were differently positioned, but the point is to not be in the middle because its harder that way(you'll get attacked from multiple ways). My tip is to load the mission until you start on the south island or north one, i did mine from the south position as seen on the map.
(red dots-my fireships)
red paint - enemy position
My strategy was to rapidly build a port and then create several fireships 5-8, and position them around the island blocking the enemy from reaching you; this way you will destroy his transports way before they eject on your island. Doing this, will allow you to peacefully build/upgrade your army to the max (although there are kinda few resources on your island - building a marketplace will solve this in a way)
Ofcourse you could start building fortresses to try defending yourself while building your army but the enemy will eventually overwhelm you, plus the map position its hard to defend because you dont know from each side will the enemy come, and the island resources are few, especially gold.
Its a hard mission, this is the first one i encounter till my 34 level when the enemy built range siege weapons (im talking about normal missions, not siege ones), and if you dont be careful for they ballista(majority are elite) they will evaporate you :). Thats why, i think my strategy with the fireships its better but everyone is free to play it their way.