Vicious fishlike hunters that stalk the shores for human prey.
Unlike most other basic enemies (Bandits, Savage Warriors,...), Greenfins are quite hardy and can take a fair amount of damage. They possess the ability to throw nets, rendering your soldiers helpless for a few seconds. There's otherwise not much strategy to defeat them, other than to pound them with everything you've got.
While the nets of Greenfins can disable the dodges of Assassins, making Knights Templar a seemingly better option, it is still advised to recruit the former, and support them with Necromancers, since they can dispatch other enemies, such as Bloodshells, Redspines and Blacksurges faster and more efficiently.