How does Odium have enough power to shatter other Shards? Although its technically possible that his percentage of Adonalsium's power than other shards, he shattered two shards on Sel. so how does he have so much power?
We don't know, as of now, much about how it was done. But it's definitely not a function of raw power imbalances between Shards.
We do have a couple of references that give some clues, but nowhere near enough to say "this is how it was done". But it requires both sufficient knowledge and sufficient power.
The best clue I'm aware of about how Splintering might be done comes from Secret Histories:
"Unraveling,", Khriss said. "So a slow death. Ati doesn't know how to Splinter another Shard? Or he hasn't the strength? Hmm..."
Khriss seems to be a pretty reliable authority on how things work, having written the Ars Arcanum for multiple settings in the Cosmere, so I see no reason to disbelieve that both of these are necessary.
Odium clearly knows how to do it, since he's done it multiple times. That leaves power, about which we know very little at present. His power may or may not be fundamentally greater than that of the other Shards, though it's certainly far less than two shards' worth of power.
We do have some clues about how Shards might reduce their own power: by Investing it into other things. Mistborn deals with this pretty explicitly, as the power imbalance between Preservation and Ruin is largely a result of Preservation Investing a portion of himself into humanity to make them sentient:
Preservation's desire to create sentient life was what eventually broke the stalemate. In order to give mankind awareness and independent thought, Preservation knew that he would have to give up part of himself--his own soul--to dwell within mankind. This would leave him just a tiny bit weaker than his opposite, Ruin. (Hero of Ages, Chapter 54)
Magic in the Cosmere has so far been shown to be a result of Shards Investing their power, so the Shards Odium Splintered may have been less powerful than him due to more investment (it's not clear when/how much Odium has ever Invested of himself, though I suspect he had not done it at all until reaching the Rosharan system).
But Sanderson has also clearly indicated that it's not a question of raw power alone.