Advanced world paramaters.
Amp requested evil, drop requested good
Increase minimum savagery and/or set savagery to be weighted towards high.
Increase the number of caves vastly of all kinds
Increase the X and Y variance for all traits (this will gen a patchwork world with many biomes) and request a huge load of titans (you get at most 1/biome)
Increase the number of curses, secrets, night creature types, evil clouds, evil rains and so on
This is important: Stop worldgen in year 2 so megabeasts don't die.
Embark with most or all of your squares covered by an evil biome -- preferably a Terrifying (Savage+Evil) one. For extra fun, take a glacier, an aquifer, and/or a coast. A beach embark with no trees and evil land on the banks of a terrifying sea might lead to having to deal with giant sperm whale thralls if you're lucky.
Tap all three cavern layers and the Forgotten Beasts will come, too.
If you're good at modding, you can edit the raws so that dragons have skin made of Slade and semimegas like ettins and cyclopses possess high (possibly even legendary) combat skills innatley. You could even take some normal creature (perhaps a semimega, but if you like fun, something common and innocuous) and make it stun-proof, feel no pain, have no biological needs or vital organs, fly, possess an intense hatred of all life, avoid traps, destroy buildings, like fighting, enrage frequently in combat, and possess a deadly dust attack that produces instantaneous necrosis of the brain.