Originally posted by Trops60:

u kill the bosses and ure raid advance up to bigger ,better raids

Actually the Ground Shaking RAID unlocks when you kill a Troll and can continue to happen at any point from then on. Same is true for several others like Skelton Surprise, Foul Smell, and Smell of Sulfur.

I've killed every boss so far on my main world and I still get Trolls and Skeletons more often than not.

EDIT: User Sarteck pointed out that I had forgotn that you also have to kill the Elder along with a Troll to trigger the Ground Shaking RAIDS. I stand corrected. :)