How to fix Kassadin

Force Pulse: Passive: Kassadin draws energy from spells cast in his vicinity, gaining a charge of Void Force whenever a spell is cast near him. Max 6 stacks. No timer. Kassadin can consume 4 stacks to use Force Pulse to deal 80/105/130/155/180 (+70% Ability Power) magic damage and slow enemies by 50/60/70/80/90% for 1 second in a cone in front of him.

Rift Walk: Cost: 100 Mana Cool Down: 5/4/3 seconds Range: 700 Kassadin consumes 2 stacks of Void Force teleports to a nearby location dealing 80/100/120 (+0) magic damage to surrounding enemy units. Using Rift Walk grants Kassadin a stack of Void Rift for 2 seconds. Max 3 stacks. Casting Rift Walk with stacks of Void Rift costs an additional 50 mana per stack and deals an additional 40/50/60 (+0) magic damage per stack.