Contract: Here Comes the Groom
You will learn about this witcher contract by finding a message at the message board in Svorlag village. Meet with Kevan (M15,1) in the island, he will have more detailed information for you. During the meeting you can bargain a better price for completing the contract. Before you start the search for the monster, agree to talk with the fiancée of the missing man - Britt. You will find her at the cliff near the village.
Now you can go to the cave in the south-western part of the island. Go inside and start exploring the nest (Cat potion will be helpful in lighting up the surroundings). In the cave you will frequently find some drowners and muckixers - use Igni on them and avoid being surrounded by larger groups of monsters. As for the exploration of the nest itself, watch the tracks left by the sirens, the scales and the large construction. Geralt will deduce that he is dealing with a melusine. You won't find the monster in the cave, so you must find an alternative exit from the location. Rocky ledges lead to the place where a monster can be found, so you will have to climb.
Before you walk outside, make some preparations - if possible, place the hybrid oil on the silver sword and check if you have some Grapeshot bombs in the inventory. After leaving the cave, find the corpse of the missing fiancée. By doing so, you will trigger the battle with Melusine.
Melusine is a flying beast. Because of that, your priority should be taking it to the ground. The regular method of using crossbow in this case won't be effective. A much better way is to wait for the monster to get close to Geralt and use Aard sign then. In the meantime, avoid air attacks (jumps to the sides!), especially since a successful attack of melusine can poison you for few seconds and make you lose some life energy. Once the melusine hits the ground, start using your best silver sword attacks or throw the Grapeshot bomb. Luckily, the monster is rather weak and should die quickly.
After a victorious battle, examine the corpse of the melusine and, most of all, take the Ekhidna Trophy. You can go back to Kevan now and take the agreed amount of crowns. At the same time, agree to inform Britt about the fate of her fiancé. You can take the gold from the woman or refuse to take any additional payment.
Main reward for completing witcher contract: Ekhidna Trophy - finding 5% more gold.