Well I do like raid map idea only. 'Sea of Monster', damn it really nice name. The boss should not same as the previous one, especially their design and mechanics, and they should be very very hard. And to make it more challenging, once all players step in and aggro the boss, the shield around battlefield will rise up and the revive will be not available until all players are died or the boss is killed.

For example I choose Captain Krom for this raid map.

Lets say we implement 'shadow clone' mechanic for him. When the hp of Captain Krom reach like 25% of it, he will unleash 4 of his 'shadow clone'. Their attributes will be like 75% from the original one. While he used the shadow clone skill, the original Captain Krom will have some boost in defense, but he will do nothing at all. Only his shadow clones will attack the players. And there will be a timer, if the players fail to kill Captain Krom once timer end, all shadow clone will be detonate, inflicting insta kill to all players.

The rest i dont bother it especially I saw gold loot thing.