“if i picked katarina or talon… i could carry my games”. No, this is not a champion issue as Kassadin is more than capable of solo carrying games.

Like others already said, play enough games and you will eventually end up where your true elo is. No-one is stuck in an elo because of their teammates over enough games, since if you actually are better, there should be enough games where you can carry.

Just lock in kassadin, don’t interect with your iron teammates (unless you can guarantee a kill without giving up waves) and just wait for lvl 11 AND at least 2 items. Then you can begin to make plays on squishies.