Yomi singlehandedly defeating The Boss's One-Winged Angel form by brandishing a crucifix and striking him down via divine retribution.
Tomo predicts that her mission to rescue Nyamo from the furries in the second season will be one of these. It isn't.
After Shin Chiyo steals Matsuda's collection of marijuana, Matsuda proceeds to beat the living tar out of her by eating some superpowered pot brownies she had cooked earlier, transforming into a tank, a helicopter, and Captain Falcon, dealing a finishing blow so incredibly powerful that it throws everyone out of the dimension. In that same battle, Snoop Dogg (who met up with Matsuda earlier) gets one of his own when he rams Shin-Chiyo with Mark's spaceship.
In Season 3, several chapters after Kagura's death, Sakaki and Chiyosuke launch a mechanized invasion of Purgatory itself to break Kagura out of the Land of the Dead. This culminates in a huge angels vs. mech fight, and Kagura shouting down God and convincing him to let her leave peacefully.
On a lesser note, even though they successfully brought Kagura back to life, new body and all, the fact that her dead old body inexplicably returned to life is, again, testament to her determined personality.
After a hard-fought battle with BLU Team, with a nuke about to blow them all away, all that remains of both sides is the BLU Spy and Ena. With all of RED Team dead and unable to respawn, they have no way to defuse the bomb. Ena, armed with nothing but a jar of Sniper's Jarate and Scout's old Pistol, singlehandedly guns Spy down, finds a way to respawn the RED Team, and defuses the bomb. Pretty impressive for a ten-year-old.
Torako making her way across zombie-infested Tokyo, unarmed, on a bicycle. And remember, this is before she became a mech pilot.
Torako walking out alone to hold off a horde of zombies advancing on a hospital filled with innocent people, and then taking down Nemesiswith her bare hands.
RED Pyro's Big Damn Heroes moment as he saves Fuuka by burning the BLU Spy to a crisp.
Tomo in the Enrichment Center arc of Season 3. Going out to rescue her friends and family without a moment's hesitation after being rescued herself, and fighting one of GLaDOS's super soldiers to a stand-still, proving that she's more than just a selfish Jerkass and that she can get over her reckless and impulsive nature and fight just as well as Torako when the situation calls for it.
Tsuruko throwing a lance roughly the size of a limosine 500 ft. up to impale Lady Kimura's Humongous Mecha through the cockpit and kill Lady Kimura.
Tsuruko toying with Emoboy (piloting a machine that barely even reached up to his mech's knee, no less) and contributing to his defeat by Kokoro.