My most badass military dwarves have all been from my current fortress, Ironstakes (from the original inhabitants, the Boots of Locking, rather than the current inhabitents who reclaimed it after the originals unleashed HFS).
1) The first was, I think, a high-level (but not elite) marksdwarf, who had a habit of carrying around about 4 bolts (at most), prefering to smack his enemies halfway across the screen with his crossbow.
One time, I got ambushed by multiple elven raiding parties (a siege in all but name), and had to shut the gates to my fortress with several dwarves still outside (I've lost too many fortresses in the past as a result of trying to get all my dwarves back inside).
This marksdwarf was one of the ones trapped outside, where he got jumped on - and proceeded to slaughter - at least one, possibly two ambush parties, got shot by an elite bowman, survived, attacked the bowman, wounding him and scaring him off, and caused several of the other ambush parties to rout.
Eventually, he became too tired and injured to fight any more, and there was still an group of elven bowmen sitting outside my gates (my remaining military was too weak to take them on). So in order to rescue him, I decided to dismantle part of the wall at the rear of my fortress, near the magma pipe, so he could get in the back entrance (which was heavily trapped and guarded to keep imps out).
He eventually crawled his way round to the back of the fortress (with the elves oblivious to the new way in), but when he was almost home, a fire imp popped up, killed one of my haulers, and then set my heroic marksdwarf on fire. Who then killed the imp, and - still billowing smoke and flames - crawled / was carried back to into the fort.
By the time he inevitably died, he had litterally four complete pages worth of injuries.
2) Another soldier from the same fortress subsequently achieved even greater badassery. Initially trained as a champion wrestler, I then equiped him with an obsidion sword and a full suit of iron plate. He made quite a few kills against goblin and kobold thieves, but his heroic career proper didn't start until (as with the previous dwarf) he got trapped outside during an ambush by multiple elven raiding parties.
As before, my initial thoughts were "Oh, crap. Another elite soldier wasted". That was until he single handedly butchered two squads of elves, and, despite apparently being exhausted, chased the remaining party all the way to the edge of the map.
I made him leader of a squad of four elite soldiers (one of whome became his lover, and was the third of my "ultimate badass" dwarves, described below. This squad became iron fist of my fortress, and was the main reason why the main theme of my engravings was elves being struck down / laughed at by dwarves.
After several very successful years, my miners one day delved too deep, and unleashed a horde of tentacle demons. About 10 civilian dwarves (including my mayor, and several legendary craftdwarves) were killed before I could send help, but my ultimate badass hero - now wielding a fire imp bone artifact sword (decorated with hanging rings of elf bone and menacing with spikes of elf bone) rushed into the fray, killing multiple demons (his kill list only counted 2, but most of the dead demons were really his handywork, with others just getting in the final blow).
Alas, his tremendous feats of arms were not quite sufficent, and he was too badly wounded to prevent the sole surviving demon from reaching the main factories and dormatories of my fortress, and killing off all the still-mobile dwarves (as well as many of the wounded).
After all his heroics, he met and ignominious end, dying of thirst in the depths of Ironstakes, as there was no-one left alive to bring him water.
3) My third ultimate badass was the lover of the previous dwarf. Originally trained as an elite wrestler, I then armed were with a battleaxe. Her first great feat of arms was almost single-handedly killing a titan. (Although thsi wasn't that great a feat, as the titan himself was rather pathetic. He had caught a hunter, and although the hunter fell unconcious pretty quickly, the titan took several minutes to finish strangling him to death, and by the time I had decidied it would be worth sending a rescue party (only one of which turned up), he had exhausted himself.
Her greatest acts of heroism were during the demon invasion, where she successfully tanked five tentacle demons simultaneously.
Alas, she too recieved debilitating injuries, and while she eventually managed to crawl into the hospital, there was no one left to tend her, and she too eventually died of thirst.
4) A non-combat example, from a different fortress. One dwarf, a leather worker, got shot/stabbed during a goblin ambush, losing a lung in the process. He then spent several months/years in bed, completely failing to recover. Then one day he got a strange mood, crawled out of bed, and spent the next month or months crawling around the fortress (or, more often than not, passing out), collecting items for his artifact, and eventually succeded in making it (a rather boring rhesus-macquac leather amulet).