Even in the narrative when they are blasting him with the power to wipe solar systems, in the very next panel after Anti-Monitor is unphased.
"But the Anti-Monitor has absorbed the total energy of his entire universe.", was the narrators explanation as to why the attack meant nothing to him.
He still had a ton of power, just not enough to destroy the entire multiverse in one shot.
So he drained them, and explained,
"There, your own energies shall change the course of history-- for only here, before creation can the future be changed. When Krona taps the cosmic forces, opening the door between matter and anti-matter... It shall be my hand he sees! My hand which shall destroy the positive matter multiverse for now and forever."
He clearly had a lot of power, it's in the books, and on the panels. Don't ride one phrase of him saying he had expended his power, meaning he had NO power, when he clearly displays he does right before he says it. He just needed more, then explained why.