Well to be honest, Shulk's combat "Arts" as they are called in Xenoblade, are not THAT special. They are mostly standard attacks (like Back Slash is just an attack that deals more damage at the back) that he uses at different positions to affect the foes. He can do a bit of healing similar to Isaac with his Light Heal. Some of the advantages of his normal arts will be negated since this is a one-on-one fight, he won't be able to go behind Isaac or to his side.
The Monado Arts is where he shines. He can use the various arts to deal various buffs to himself and his party in the game. Speed for becoming exceedingly fast, Buster to just deal a lot of damage (but less damage on non-Mechon), Armour to buff up defense, Eater could remove Isaac's buffs if he gets any, etc.
Some of his Arts will be useless. Plus all of his Monado arts require a period where they are charged, i.e. he has to attack with standard attacks till he is ready to use Monado arts.
Being able to see the future doesn't mean much. Just because he can SEE it, doesn't mean that he can PREVENT it. There are many events, storyline wise as well where Shulk is unable to change the future just because he saw it. Even in a standard fight, the visions can come true, even when he already saw the future (i.e when you lose a fight).
Isaac on the other hand has a HUGE variety of moves. Don't need to describe them as this is the Isaac thread after all. Plus they are destructive on a big scale. Shulk doesn't really meet such types of moves in his own game so Isaac's moves are on a different power scale. Plus he has Djinn to boost up his stats and the catastrophic summons.
Shulk relies on the power to control Ether (the building block of the world + life) and "technically" change his surroundings. It is not really seen in Xenoblade much except near the end. I don't see the battle tipping in his favour because of it though.
In my opinion, Isaac can win this battle. With his power to manipulate Earth and summons and what not, Shulk will have a hard time keeping up with his visions (oh the migraines
), but be able to keep up for sometime with his Monado arts. His Shield and Armour can boost his defensive capabilities considerably though while Speed will boost his attack speed. He won't be able to have multiple buffs at the same time. Isaac will win if he manages to make a definite attack (like Summon) before Shulk is able to use a Monado art to protect himself from damage.
This is just my opinion though on how I perceive the power scales of both games. I hope I didn't talk too much Xenoblade Jargon,
and didn't give way any spoilers. Feel free to shoot down my argument Xenobalde+GS fans if you think I got it wrong.
In my opinion, Isaac needs foes that can change the world or have largely destructive powers. He may at be at a disadvantage against them, but the battle would be much more engaging.
You guys are lucky Sakurai likes Japanese RPG's like Fire Emblem, Xenoblade, etc.
Isaac fits into the criteria of games Sakurai seems to favor.
It might fit in the criteria but so far Sakurai has either not noticed Golden Sun (next to impossible) or it has not grabbed his interest enough like FE and Xenoblade
Or he just didn't like it as much as the other 2. Otherwise we would have had a playable Isaac already. Sakurai's bias is big enough to do that at least.
Ryan from BrainScratchComms revealed Isaac is his most wanted character
Cool. More exposure is always nice.