In the Metal Saga it's typically shown that the bladers are only really represented by the beys, and that the power of the Bey = the power of the blader themselves, to the point were Ryuga can even defeat a group of mooks without even letting L-Drago out of his gauntlet. That means Gingka's power is equal to that of Pegasus, and at it's strongest, Pegasus would be planet-star level.
As for how Midoriya could win this? Smash the ground. After all, falling to his demise is what killed Rago, Pluto, and Doji. Gingka wasn't able to surpass Rago without using the power of all blader Star Fragments, which wasn't a permanent buff.
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