Hm, sonic's invincibility is timed, while Ike can only be hurt by weapons blessed by Ashera. And in order for sonic to get Ike's weapon, he needs to pry it from his hands and then make a getaway with a humongous sword in tow. As Ike cannot be hurt, sonic would need actual physical strenght to get his sword from his hands. Super sonic only grants invincibility, not super strenght or the likes. Even if sonic managed to remove Ike's sword, he wouldn't get anywhere with it.
Lol Ike's like ganondorf. <_< Invincibility against everything ftw. At least his sword is removable... but one needs lotsa physical strenght or special tricks to be able to do that. How great is tornado spin btw? Does it create a tornado of shorts that would loosen Ike's grip, even if he didn't get physically hurt by it?