The very first acts of Guardians was to banish nearly all magic from universe into starheart. Originally told in GL v2 111 and recounted in GL Corps Quarterly 7. In GL v2 112 its stated to be able to scatter through Omniverse.
Here it is shown that the Guardians defeated the magic users across the universe and then banished the magic.
Guardians imprisoned Parallax entity in the CPB. Its notable as Parallax is one of the fundamental forces of the universe. Green Lantern rebirth 3.
The guardians defeat the living sector 3600 and compress the whole galaxy to the size of a ball. Tales of GL Corps annual 2.
Long ago, Guardians defeat the entire race of Djinns and imprisoned them in their bottles. Legion of Superheroes 267.
They also defeat the Emerald God of Ekron and seal his power in his two eyes. Legion of Superheroes v6 annual 1. Just one eye of the god was able to warp reality on planetary scale.
Appa ali apsa or Old Timer transports entire cities from all over universe. Every galaxy GLs patrolled.
Kept the entire planet stabilized by his own power.
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Casually no sold Hal's attacks who was powered up by CPB and beats him down.
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And then he absorbs the entire power of CPB.
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He beat the Guardians because they didn't know how to fight and engaged in him with two on one.
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But then, just three of Guardians beat Old Timer with the entire power of CPB and killed him.
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Young maltusians were so powerful that one stray thought could kill entire races instantly and wipe them from reality. They could also levitate mountains with ease.
Ganthet shields himself from the same entropic energies which birthed the multiverse by duplicating Krona's experiments.
Shields Hal and then replenished his life by giving his own life force while two other Guardians were killed by Entropy.
Green Lantern: Ganthet's Tale
A controller (equal to Guardians in power) created Triarch who were capable of thrashing entire GL corps, Darkstars and LEGION.
Turned out they are controlled by the Controller who takes on entire GL corps and Darkstars simultaneously and kicks all of their asses. He is only beaten when his control fails and the Triarch turns upon him and later turn the entire planet into paradise.
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When John Stewart is given the power of a Guardian, he resurrects Katma Tui, defeats several GLs including Hal, Guy, Kilowog and JLA (J'onn, Power Girl and Flash) and a world destroying alien armada together, shows omniscience (Knows what's happening on Earth from Oa) and omnipresence etc.
Guardians reveal that they had planned it all along.
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Green Lantern: Mosaic 18.
Lianna, a child like Guardian ages herself to adulthood. Then she casually shitstomps Kyle and Jade and lays waste to an entire planet. Ganthet fixes all of it with a wave of his hand.
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Child Guardians and Ganthet stop a barrage of quantum singularities, each one capable of destroying entire suns (and specified as objects of infinite gravity). Absorbing just one such singularity nearly killed Kyle.
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Xadai (Kilowog's demons in afterlife) surprise Ganthet and overpowered him. Later Ganthet rips open the realm of afterlife and after an amped Kyle and Kilowog as Dark Lantern (Someone powerful enough to stomp Justice League, Sentinel and even cut Zero Hour Parallax in half) struggle with Xadai, Ganthet beats them with a literal clap of his hands.
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Dark Lantern scans below:
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Krona's mere presence on Earth throws Moon out of orbit and entire Earth is thrown into chaos.
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Guardians defeat Krona and reduce him to energy once again. Krona's psychic imprint lets Hal see everything in creation at once and gives him power to fix everything too.
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Now comes a Guardian's most famous showing. Krona from JLA/Avengers.
First, Origin of Krona. Its mistaken that he was powered up before he was destroying universes but that's not shown anywhere. He is your average Maltusian before Guardians evolved into what they were today.
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Krona destroys two universes.
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Grandmaster blocks his probe and Krona manifests and reveals he had absorbed the power of the universes he destroyed.
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After Grandmaster wins the game, Krona casually stomps him.
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Casually stomps Galactus and kills him. He even made his fortress out of Galactus' hide. Ultimate ownage.
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Starts merging universes after Grandmaster used twelve items of power to bind him between universes. Killed Grandmaster and usurped the power of the artifacts of power.
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Now another of the greatest showing of Guardians' power comes from Zero Hour. Remember at this point CPB was just a focus for Guardians' own power as shown in Secret Origins 22.
Hal absorbs the power of the CPB.
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Hal had absorbed chronal energies left after COIE to destroy time stream. Here he destroys an entire universe by opening an entropy rift. Later Extant says there are other entropy rifts he was not responsible for.
Hal kills the Time Trapper with his own power after he survives entropy hence showing the power of Guardians was greater than Entropy itself.
Hal explains how he absorbed some chronal energies to destroy timestream.
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Recreating the reality was from Guardians' power and knowledge as stated.
And he actually destroyed entire creation destroying infinite timelines.
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Then he starts creating his own creation using the power of Guardians. You will notice the green hue in the globes of energies which were solidifying as entire galaxies.
Spectre and the heroes fought Parallax, the heroes absorbed the plasma universe. Hal defeats Spectre but the effort lefts him weakened and the heroes prevailed.
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In the official recounting in 52 issue 7 it is stated that Hal was creating a fully fledged multiverse until Spectre stopped him.
The remnants of power left within Oa after it was destroyed by Kyle Rayner give Thanos enough power to destroy both DC and Marvel Universe.
Thanos with power of Oa vs Parallax with power cosmic.
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We know its canon for DC as Parallax mentions how he travelled to another universe in pursuit of Cyborg Superman in Parallax: Emerald Night.