Mercury versus Malleus. Mercury doesn't know how to make Dungeon Hearts? Why not take on an experienced Keeper who is down to their last dungeon heart, and who has turned their Dungeon into a giant meat grinder designed to wipe out invading armies? Oh, and this is done in the second week or so of Mercury being a Dungeon Keeper.
Mercury's Azzathra-sponsored duel with the Reaper. Alone, in an arena designed specifically to disadvantage people like her and prevent any clever trick she might pull, against a reputably unstoppable Lightning Bruiser, with her advantages methodically stripped away from her one-by-one. A "duel" that was, for all intent and purpose, a particularly showy public execution. And yet, despite it all, she starts her tradition of Defeating the Undefeatable.
Mercury versus Zarekos. An ancient vampire and the sole Keeper of the Avatar Isles, he was an extremely powerful foe. Her solution? Scry-and-die. Teleport behind him mid-battle during the night, catch him, teleport him to the other side of the world where it's the middle of the day (and above the ocean, thus combining two vampiric weaknesses). Sure, he had contingencies, but still an amazing victory that turned the tide of the war. The fact that she was dealing with several other threats simultaneously didn't hurt.
Nero casting the Armageddon spell, thus teleporting all of Ami's minions to his base for a fight to the death? Check. Keepers Arachne and Alphel both using that opportunity to attack Mercury's base, while only she can defend it? Check. Alphel boosting herself for that fight by possessing a legendarily gigantic, unstoppable dragon? Check. Alphel making what was already an amazingly unfair fight even worse by the use of a dark god-given anti-magic stare? Check. Ami still finding a way to win? CHECK AND MATE.
Why hello there, Avatar! Utterly unstoppable, ridiculously powerful champion of the gods? You don't say! Able to shrug off any attack, any spell, any trap Mercury throws at you? My my! A legendary One-Man Army that gives Keepers nightmares? Very impressive! But...you've never heard of electromagnets, have you? Mercury has.
A moment of awesome for the Light happens after the Avatar's defeat when the Avatar sent Ami a horseshoe magnet. Knowing what actually happened in a world of people and gods used to explaining everything by "That's Magic" is a feat in itself.
Similarly, the Unraveller. "clever application of lightning" indeed
The Incarnation of Extinction, a Lesser Aspect of Crowned Death. Totally impervious to all non-spiritual attacks. Beat the snot out of a Horned Reaper. In a position where it could potentially incarnate into the material world, allowing Crowned Death to directly influence it. What did Mercury do? She absorbed all of its energy. She didn't just punch out Cthulhu here, she ATE HIM.
Ami's attempt to dominate one of Crowned Death's undead dungeon hearts in its underwater temple resulted in her being attacked mentally by Crowned Death. Enter her sister Tiger, who possesses Ami, uses that link to connect to the heart, and orders the heart to flood the underwater temple with chlorine triflouride. The result: the stone temple sets on fire. The sand of the ocean floor it crashes into sets on fire. And the water surrounding the temple explodes. This destroys the temple, obliterates the power source of a set of ancient magical artifacts used by Crowned Death's priests, and kills a second Incarnation of Extinction.
During the same chapter, Torian actually growing a pair and using his newly-devised Dominate Undead spell to take control of a death priest and using its own rapid-aging spell to disrupt Crowned Death's control over Mercury through his incarnation. It ultimately fails, but it's a good showing from a character whose biggest contribution until then was kissing up to Ami and accidentally providing her with ideas.
The evacuation of the Avatar Isles. Multiple Dark Gods join forces to destroy Ami, special corruption is destroying her dungeons, a massive army is closing in on her lands. Not only does she survive, but she uses the Armageddon spell to teleport the invading army into her abandoned base...right before blowing it up with massive amounts of explosives.
With Mercury getting so many Crowning Moments of Awesome, Mukrezar feels he has to remind the world he's still the most dangerous Keeper around. So, working together with Crowned Death, he gathers an army of the Dark God's intelligent undead and destroys them them all in a single decapitating strike, crippling Crowned Death's forces to an even greater extent than Mercury had - and it's implied that he did it to show Mercury up, but also to get revenge on Crowned Death, and because at least one other Dark God was paying him for it. Magnificent Bastard indeed.