Heres my solutions i can give via other people.
1) There are many ways to kill a Bronze Collosus. For starters:
A: Cast out a long drawbridge, and should you have a magma area, or perhaps lucky enough, a water source, you can flood enough of a channeled hole to keep it out, and keep a lever long inside the place so they don't "run away while trying to lever it" and he will flip into the air, and fall into the water/magma. Though water won't have an effect, if you were to put a 30 z-level place under the bridge, it should kill him when he falls down. Hell, i bet 100 z-levels will be overkill, and worth it, but way too long without at least 3 miners.
AA: As a side note, you can instead make pressure plates on the starting area of the bridge and set them to one-use in order to make it stay up, but keep a lever for putting it down, slate it for removal when you put it back down, and rebuild it.
B: The atom-smasher move works also. But i'm not sure how it works.
C: If for the love of god, you can perfectly make a region with a volcano, some trees, and water, you can easily prepare a
Boatmurdered strategy, aka, Flood the world with magma, or more specifically, the map itself. You can easily flood with pipes enabling you to immediately kill the Bronze Colossus before it makes it to your entrance. I actually want a region that enables me such a feat, but i can't find the perfect seed. If i did, whoa nellie, i will rip all invasions to shreds, and actually survive even a megabeast. But underground attacks... are horrid.... unless you wanna pump magma down to the underground to stop them via stair removal, channeling, and pipes.
2) Well, i can't say for sure if you can stop it from moving to you, but i assume you can edit the parameters on titan requirements to 10,000 population. But i'm not sure if titans are megabeasts. I wish you could easily set how many megabeasts there are, and of what kind.
3) I was 100% certain i could easily know how to remove it from your region entirely
without messing with the raws, however for the god of it all, i can't remember.
......
Theres plenty of ways, but... i guess everyone already answered all these... i didn't feel like reading everything on all posts. Sorry.