Feathertail kills a mountain lion with a stalactite.
To be more specific, she impaled it with the stalactite.
Cinderpelt's defense of the nursery during the badger attack is pretty awesome as well. Cinderpelt is a medicine cat, although trained in basic self-defense, she's not expected to do any actual fighting. On top of that, she's crippled by a twisted hind leg. Yet when the badgers invade ThunderClan and move in on the nursery, she barricades the entrance with herself and fights them off. She dies in doing so, but StarClan rewarded her with a second chance of life, and reincarnated her as her own niece.
Tawnypelt gets hers when she rejects Tigerstar's offer to gain power, saying she'll gain respect and power her own way.
Hawkfrost puts Brambleclaw into a What You Are in the Dark situation—tighten the fox trap around Firestar's neck, taking the rest of his lives instantly, and becoming leader himself. Brambleclaw effectively tells him to screw himself, then, when Hawkfrost attacks, kills him by driving the same stick from the same trap into his neck. Bad. Ass.
StarClan put an end to Mudclaw's rebellion by dropping a tree on him, also allowing the Clan to cross the lake to the Gathering island at the same time.