Chapter 16: Wrex fights Avatar State Shepard to a near standstill.
Chapter 17: Shepard vs Sovereign and Saren. Both of them.
Chapter 19: Shepard fights the Sovereign-possessed Saren with her crew's aid.
Chapter 31: James Vega and Anette, an Ardat-Yakshi, invade one of the Collector Ships and run into Leviathan. At first, he has the upper hand... Until Anette melds with him, claims about 10 million of his composite souls, and proceeds to singlehandedly kill everything else on the ship till Leviathan stops her, and then prevent it from crashing after Leviathan cuts part of it away.
And in a lesser CMOA, Leviathan then, despite his portion of the ship not having engines, apparently uses his biotics to accelerate to FTL drive.
Chapter 37: during his Trial to become a shaman, Grunt encounters a Thresher Maw. Not only does he get it to back down, he tames it.
Chapter 43: Morinth attempts to eat Shepard's soul... and discovers, in rapid order, that the Avatar Bequest is a Defense Mechanism Superpower, gets pummeled mercilessly, and biotically fired into the Omega-4 Relay.
The attack on the Collector Base. All of it. Special mention goes to: