VERSUS

I was wondering if the Mane Six would have any prayer (no pun intended) of defeating the Giygas of Earthbound with only the Magic of Friendship and the Elements of Harmony.

...Let's discuss it, shall we?

Personally, I think the Mane Six would be just as (if not more) screwed as the Chosen Four prior to enlisting the succor of a higher-planar entity (the will of the player him/herself) through "prayer." Yes, it's true that the Elements of Harmony defeated Discord... but, Discord was only shown to be an interplanetary threat, at best capable of messing with the solar system, and lacks any feats showing greater abilities than that. Meanwhile, Giygas is the Universal Cosmic Destroyer confirmed to have threatened all of existence, and would have destroyed the universe where it not for Buzz Buzz going back in time.

Furthermore, Giygas is not an concrete physical entity like Discord, but rather an incomprehensibly non-corporeal dimension that exists outside of space and time, even when in the Devil's Machine, forming the abstract embodiment of the concept of evil. So, it's not as though the Elements could turn Giygas into stone, banish him to the moon, or contain him somehow—he is evil itself, so he is omnipresent as long as evil exists. I would personally wager that, in terms of threat, Giygas is at least a few magnitudes above Discord's tier.

Despite having effectively become what could be basically considered the universal nexus of positive psychic energy through defeating his own evil and absorbing the Magicant, Ness is unable to deal any notable damage to Giygas himself, proving that all the prophesy about his destiny being the defeat of Giygas was worthless. Even the formidable PK Rockin', the psychic embodiment of "joy, hope, and pure will" empowered by the concept of "Ness' favorite thing", could not make a dent in Giygas' might. If the nexus of positive mental willpower didn't have a chance of defeating Giygas, I just can't see the Mane Six faring well on their own against him. Besides, Equestria Girls showed us that the Elements of Harmony themselves are not unapproachable to evil...

How can we expect the Elements of Harmony to fare any better than the pure hearted icon of good Ness if they can be so easily manipulated towards evil ends? Even if you don't count Equestria Girls as canon, I just don't think that the Mane Six can succeed where the Chosen Four could not.

1367609 I'd say that your argument makes sense. Although it may just be because I'm exhausted and may overlook some sort of flaw if it exists.

1370547
Sleep on it and come back to it later, then.

They have no chance