How can I protect myself against being thrown off the islands?
Solution 1:
Set up wall spells behind you.
Don't use a shield.
Throw them off before they throw you.
Pray.
Solution 2:
While I agree with the foreanswerer on some techniques, there's one that works wonders if you have access to it - teleportation.
It can be achieved by either the Teleport spe-- cough... Magick... or the Staff of Emergency Teleport. Since you're in the sixth chapter and the Magick is unlocked in the seventh, try finding a Goblin Shaman for the safety measure this staff provides.