Luke Skywalker.
EU Luke is a beast. Someone else pointed out how Starkiller yanked down the Star Destroyer. Luke is way, way above Starkiller. Granted, the SD was already crashing and Starkiller just guided it, but still.
EU Luke is also insanely fast. I have no doubt he could dodge Sovereign's fire.
Sovereign's arrogance would cause him trouble here, I think. Luke's gonna seem insanely powerful and we know the Reapers are interested in humanity. With Luke firing off Force powers that aren't anything Sovereign can recognize, I think he might try to capture Luke. I mean, the Reapers understand biotics, but the Force would be completely foreign to them.
The biggest thing here is that Luke is very skilled at planning on the fly.
Oh, and this.
http://starwars.wikia.com/wiki/Shatterpoint
If he can find a weak point on Sovereign, then Sovereign will at the very least be severely damaged.
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Gamertag-Murphiroth
Pokemon Black-1978-9490-1161