Where do you find the magnet needed for atlantica?

Once you complete Halloween Town the first time, you receive the Magnet spell.

Strategies for Xaldin haved been answered before in previous questions. Just check the previous pages of the Questions section.

To defeat Storm Rider in Land of the Dragons (2nd visit), you need to attack the horns on it's head. If you are already on it's back, use the reaction commands available to prevent yourself from falling off. If you do fall off, use the Ride The Wind reaction commands to ride the air currents back onto it's back.
Use the reaction commands when it slides on the ground, when it's trying to ram you. And avoid the giant lightning attack when it powers up on the ground by moving to the side. After this attack, you can jump back onto it's back.
And remember to keep on moving if you're on the ground, to avoid any minor attacks coming your way.

To defeat the Grim Reaper in Port Royal (2nd visit), you need to put all the medallions back in the chest, then attack it. All you need to do is collect medallions lying on the ground, and use the reaction commands when available. Remember that in it's undead form, you can only hurt the Grim Reaper by using magic.
You may also find the Draw and Quick Run abilities useful.

How do I find the magnet in the little mermaid world Atlantaic