I remember reading on the Wiki that Bronze Colossi suffer from cumulative damage.
So as long as you keep denting and chipping it, it should go down eventually...
This is a true thing. I had one fort where I was trying out danger rooms, and had cruddy copper equipment for all my dwarves because it was the only thing on the map. Had, I believe, 4 10 dwarf squads, or thereabouts. Anyway, the colossus comes, and I throw my army at it. I'm dealing damage, and putting multiple body parts into the red, but the thing keeps crippling/killing my dwarves. Worse, this was before the healthcare fixes.
Eventually, almost the entire military was dead or hospitalized, except for about 4 melee dwarves and 6 or 7 marksdwarves. The only thing standing between them and the colossus was the fact that said monster had leapt into one of the various ponds scattered about.
So, except for the thing scaring my dwarves despite now being completely harmless, life returned to normal in my fort for about 10 minutes in real time, when a minotaur showed up. I send out my ragged, war-torn squads, and focus on the minotaur. My melee dwarves show up (bolstered by a few choice recruits), and start getting the pants beat off of them. After a bit, I realize my marksdwarves aren't shooting him, and what remains of my melee squads was being beat to death by a cow man.
I went to see what they were doing, and they had just started making their way toward the minotaur. Satisfied, I resumed my vigil. So then they run up and beat on the minotaur with their crossbows, and start getting beat on in return. Fantastic.
On a whim, I go to check my deceased list, to see how many dwarves had actually been lost, and I see the colossus on the list. I checked the reports, and found that the marksdwarves had expended all their ammo on it, which was why they were meleeing the Minotaur, apparently.
Apparently, one of the crossbow bolts severed it's upper body.
I was unfortunately unable to recover the statue before winter set in and froze it into oblivion, and the fort was soon steamrolled by a goblin siege backed with trolls, with all my dwarves in the hospital. A couple cage traps thinned their numbers, but my civilians were helpless.
Never going to forget that colossus though. Unlucky bastard.
TLDR: I nickel and dimed a colossus, eventually severing the torso with a friggin'
crossbow bolt.