Finish the first two archipeligos and hit 'RETURN' to select another.
Type "8421" and hit 'RETURN' twice. You can now go wherever you want. (Possibly do it after booting.)
Enter "1" to "999" followed by 'RETURN' key to select island.
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.