Firstly, you have to remember that all the minion rocks have to be connected by land to the monolith. This doesn't have to be a direct route, but just so that you can trace a path from rock to obelisk without crossing water. The easiest way to do this, when searching for the stones, is not to jump over the small stretches of water you come across. Deposit a bit of land and then cross. That way, before you know it, you've linked all the islands in the archipelago.

Something else worth remembering is that the stones are widely spaced. You don't have a time limit for finding them, so wander around for a bit. Note the position of the stone nearest the monolith and destroy this one last. That makes your job of destroying the monolith within the 90 second time limit a bit easier.

The easiest way to get rid of the necromancer is to build a piece of land, about three squares long and one square wide, jutting out from the coastline just before where the necromancer will strike. He'll teleport to another section of land, after a delay.

Try to find the blood eggs before they hatch. When they burst, they spread their poison to all of the connected islands, even through the sand and land connections.