Think about what CHARNAME achieves in the defeat of Yaga-Shura - s/he single-handedly (well, up to six-handedly) broke through the lines of an entire army and defeated its leader, who up until mere hours before their arrival was literally invincible (and few people would have known that that was no longer the case, up to and including Yaga-Shura himself). The stuff of legends, indeed.
Indeed the PC has multiple opportunities to take on entire armies in Throne of Bhaal. Cast Timestop, three "Horrid Wiltings" and rejoice as platoons of drow spontaneously combust into clouds of splattering gibs!
From the popular Ascension mod, going up against Abazigal's entire family of dragons — at the same time — and winning (ornot, as the case may be).
Defeating Amelyssan at the peak of her power no doubt counts. Bonus points for playing the Ascension version of the battle, where you get to defeat the rest of The Five (or four of The Five plus Gromnir if you take a certain option) all over again!
In the first game with certain mods installed, the battle against the Chill and the Black Talons mercenaries in the bandit camp can become very difficult so, winning the battle is one Moment of Awesome without a doubt! Your party who may made up of just Level 5 or 6 adventurers just beat a small army.
In the sequel, when you infiltrate a Drow city in order to both learn more about Irenicus' alliance with them and save a silver dragon's eggs, you're able to talk down an Aboleth that threatens to expose you.
In BG2, most of your party members throw out some pretty good lines before the final battle with Irenicus (including a surprisingly good one from Aerie).
Minsc: I grow tired of shouting battle cries when fighting this mage. Boo will finish his eyeballs once and for all, so he does not rise again! EVIL, meet my sword! Sword, MEET EEEEEEVIIIIL!
Jan Jansen (Yes the Cloudcuckoolander gets one): Some villains refuse to die. Kill 'em once, kill 'em twice, they just keep coming back. It's just like a bad play. Here's hoping for a decent ending...
Jaheira: No more platitudes, Irenicus. You have taken much from all of us. Now you die your final death!!
Edwin (in one of his rare Badass moments): Power is on our side, sorcerer! You cannot hope to defeat us in this final reckoning! Your end is near at hand, wail if you must!!
Aerie takes several levels of spunk in Throne of Bhaal, but especially if she's in a romance with you. Midway through her romance, she becomes pregnant with CHARNAME's child, yet opts to keep travelling and fighting alongside you. Better yet, wait long enough and she'll give birth, quietly patch herself up with a few healing spells, and continue on as normal. Pregnant Badass indeed.
Even better: The baby itself is an inventory item. That means by the end of the game, you are invading dragon's lairs and hellish demiplanes with an infant strapped into mommy's backpack.
Deciding to stay mortal instead of ascend to godhood is pretty Badass.