One of the quests in Fort Finch requires you to kill 3 Leader Grunts on the island, but a lot of people have trouble with actually finding these. The thing is that they are kind of hidden, and in order to find these three you need to create a pathway around the island to get to a small patch of grass behind the main castle.
I found the best strategy is to bring 50+ Dirt Blocks, and just follow the left of the castle walls for as long as you can. Once you can't progress any further, begin to make a row of Dirt Blocks to enable to you travel further, and you will soon see the small patch of grass where the Leader grunts are located.
It is also possible to jump down from the castle wall to land on the small grass patch where they are, but I find this to be a bit more difficult, since you can't really see them from above. However, this is entirely possible if you find out where they are.
The fight itself is nothing special, but make sure to be careful, since it's very easy to fall of the island if you rush in. The three Leader Grunts are also located on a rather small patch, so you might have to fight two or even all three of them at the same time, so bring a health potion or two.