An atom smasher is simply a raising drawbridge. When it goes down, the things under the bridge get smashed (some exceptions are Titans/FBs, Clowns, and very large creatures, such as Rutherers). Raising drawbridges also smash things that are in the "hinge" tile when raising, while stuff on the extended part of the raising bridge gets flung in a somewhat random direction.
In order to get the bridge to smash things you'll have to get them there, which is typically done by putting a dump zone where the bridge lands and designating undesirable stuff for dumping (there are other methods, such as using water to push items). Note that you cannot put a stockpile on top of a bridge, but you can have a track stop dumping mine cart contents onto a bridge. However, if the bridge is accessible (not down in a hole or so), dorfs tend to pick up the stuff just dumped and haul it to a stockpile, typically the one feeding the mine cart that dumps stuff onto the bridge...
I've never seen a bronze colossus or any other mega beast, just semi mega beasts and FBs/Titans. If ½ year is in game time you're way too impatient. There is a trigger Titan attacks that defaults to a population of 80 (can be set to 50 and 20). FBs use some other condition (if any) but may take 2 or 3 years before they start showing up, but can really appear any time. Note that FB and Titan are the same thing, but Titans appear above ground, while FBs appear in the caverns.
Goblin sieges are set to a trigger pop of 80 as well, by default, by the way.
Butchering and evil biomes: Note that it's only reanimating biomes that make butchering risky. Other, milder forms of evil biomes are safe from a butchering perspective.