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 is, when searching for the stones, 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 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. 

Now for a couple of odds and ends. The easiest way to get rid of the necromancer 
is to build a piece of land, about three squares long and one wide, jutting out 
from the coastline just before where the necromancer will strike. He will 
teleport to another section of land, after a delay. The lost souls are 
indestructible, so give them a wide berth. Don't worry about running out of 
energy when creating land, as the energy rocks are in abundance. Finally try and 
find the blood eggs before they hatch. When they burst they spread their poison 
to the whole lot of the connected islands, even through the sand as
well as the land connections.