[@thewizardguy]
Swords can't slice planets in half. Measuring his attack power isn't really fair, since the larger part of his damage is non-physical, but as he was able to physically harm Tamamo during part 2 I would say he is at least mountain-buster level in terms of raw strength, using Gnome. Probably quite a bit higher.

Of course, that doesn't actually mean he breaks mountains. That's not how swords work. But, his blows most definitely contain the strength required to "break/topple a mountain". 

Quadruple and Sextuple Giga both leave pretty much nothing where they are directed. They reshape the landscape, and leave a massive rift across the cutting path of the blow. Quadruple left a massive ravine across the field where he first used it, and future uses were more than capable of demolishing large sections of a castle. I have no doubt in my mind that Sextuple Giga can dissect a mountain, levelling parts of it in the meantime.

Sword of the Heavens is a good deal stronger than Quadruple Giga, but it's more concentrated, so the end result is less destruction - more damage. Swords aren't made for big, destructive attacks. The same goes for Daystar, which is non-physical, but is more or less on par with being hit struck by several meteorites of comprised of holy energy.