Kassadin is not equipped to fight against the Void.
So either he needs a rework ('cause not only does the kit not fit his lore/theme, but it's also a kit of the design that it's either too weak or too strong) Or they need to fix the lore to state that one of his goals is to search for allies that an fill in the gaps in his strength, 'cause trying to fight them all by himself is pretty lame when he can't even manage to make a shield that blocks CLAWS.
Passive: -15% Magic damage Q: Magic-only shield and an interrupt that only works on one Void champion in a 1v1. W: Is ok E: Requires spells spammed nearby, which often means taking a full burst to the face before it can be used. R: Only one voidspawn relies on attacks that can be countered by a blink.
So the kit is designed around fighting magical opponents. Makes sense since Cho'gath was the only Void champion in the game when Kassadin was released.
But
Physical Damage Void champions: Kha'zix, Rek'sai Magical Damage Void champions: Cho'gath, Vel'koz Mixed/Either: Kog'maw, Malzahar.
Skins: 2 physical (Nocturne, Illaoi), 1 magic (Fizz)
Kassadin is thus poorly equipped to fight the enemy he's designed to fight.
And keep in mind that his relation to the Void at all was just shoved in to give some relation to Malzahar a few years later. But with the current story he'd be better suited as an Aspect than just a dude who fell into the Void.