Jamz seems to have a pretty solid Sonic.
Sonic's main focus is to punish any and all lag on any moves you see that have said lag. Sonic's ground speed allows him to punish anything of this sort with terrifying efficiency.
Having said that, Jamz needs to work on his ability to read and punish. A lot. There were quite a hefty number of opportunities during that set where Sonic had a guaranteed grab (one of Sonic's best damage builders in that it puts most opponents in bad aerial positions) or smash. There were also scenarios were he should've applied more pressure to the Dedede, but failed to do so, and gave the Dedede room to breathe.
That, and he shouldn't be getting hit by Dedede's dash attack or sideB more than once.
He also, based off of the first match, tried way too hard to land smash attacks at near 0%. While this is all fine and good when they land, it's not something that a Sonic player should rely on for early damage. Sonic's smashes take forever to come out, and forever to end after the hitbox is released, and when it comes to vs. Dedede, that leads into a chaingrab off of the stage which builds up percentage, and puts you in a bad spot vs. Dedede.
Anyways, those were just my thoughts based on watching the first game.
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Second match reflections were worse. He played better in that he didn't start out trying to add up % with smash attacks, but that didn't really help him when he started trying for smash attacks around 90%.
He ran into Dedede's dash attack again as well, and although D3 didn't really slip up in terms of lag as much this time around, there were, again, several missed opportunities in the set.
Overall, his performance was worse not because of this, but because he played the Dedede match up completely wrong on this level. Sonic is suppose to abuse the sideB invincibility on this level. Dedede can't do a **** thing about it. Instead, he uses it once here, once there, but opts to approach.
Had he stayed behind and forced Dedede to approach by abusing the invincibility, he just might have won that match.
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Overall, it's bluntly obvious that he was being completely outplayed. He's good, but he still needs a bit of work.