Originally posted by Bobucles:

Raid power is determined by the world. Every boss down upgrades the power of raids.
Buy the eggs and use them in a virgin world. The most deadly raids will be greydwarfs, easy to wall off and forget.

Note that you have to buy them in a world where Yagluth was killed, but you can then take them to another one. I did this once and my chickens got to peck around outside in real grass. It's a shame we have to deny the little gals their sunshine on a normal map.