I learned today that Giants can be killed with a single not well-trained marksdwarf - if he has bolts. I also learned how to equip my marksdwarves with bolts.
I had a well-running fortress, about 70ish dwarves, 5 iron-armored silver-hammer-dwarves and one little marksdwarf. And I had an entrance prepared with four bridges that was supposed to lead an invader the longer way which then would be shut off from the fortress and the outside and also divided in two parts. That was all ready, the burrows for my marksdwarf-army one z-level above was ready, too. The fortifications for the siege engine battery had been carved though I had not come around to start the construction of siege engines.
When the beast came, about a third of my dwarves were outside gathering wood and such. So I ordered everyone inside and left the entrance open until the very last moment. It really was the very last moment: The innermost bridge raised with the giant being two or three tiles away, my melee dwarves standing ready right behind it. I thought as a giant it would just destroy my feeble stone bridges, but it didn't and when the other bridges were being raised, it was caught. I ordered my marksdwarf to its burrow to tickle the beast with bolts. He had no training except for being an immigrant hunter I recruited as start for my marksdwarf squad. He did not shoot no bolt. I wondered and thought tjat maybe shooting down a z-level doesn't work after all. So I started installing my siege engines when I realized that I was out of logs. And with a giant in my main entrance, I couldn't gather any. (No caverns had been breached then and with a giant at my doorstep, I did not want to breach one then, really. :-) )
I started digging a second exit(, something I wanted to do anyway to connect my fortress to the brook in the corner of the map and shorten the way for my bucket-squad refilling my cisterns). Even before I was done, I reinspected my marksdwarf and saw that he had everything he needed except bolts. It took quite some time to figure out where I can tell him to pick up bolts, but when he finally did, it took only half a dozen shots or so and the giant was unconscious, in pain and suddenly not looking that large anymore. I lowered my bridges and my melee squad ran forward to slay the beast, but the marksdwarf got the kill with another half a dozen bolts before any hammerdwarf got even close.
Suffice it to say, when the message came that a giant had appeared, I was afraid to loose the entire fort - that would have really pissed me been Fun since this fort was the longest I had so far. I only lost one brewer who was to dumb to run inside in time sacrificed himself so the other dwarves could get in safety.