with the stranded build of maxing R>Q>E>W on Kassadin a full combo could kill them in a much slower and more clunky manor. With sheen Kassadin most of the damage comes from the W which allows for amazing one shot damage and very good mana restorations. sheen Kassadin is meant to be a get in get out kind of assassin and after maybe 2 seconds you go back in to pick up another kill. The maxing order, respectively for sheen Kassadin, is R>W>Q>E. Kassadins combo is ulting in and immediately applying your Q to the target to get a small, but incredibly useful, magic shield, following that up with a W then E so the chase can continue if you haven't already killed the target.

The spells are barrier and flash because Kassadin has enough potential to kill without ignite which makes it a useless spell late game. Barrier allows for extra protection as you are gonna always be the center of attention especially when in the face of the enemy teams ADC and Mid Lane carry. Though Kassadin already has a built in flash you don't know when you're gonna get into too sticky of a situation to wait for your ult cooldown to complete. The runes are meant for good early game cooldown and AP.

Sheen Kassadin has a build that is orientated around CD and AP. The Rod of Ages is a good starting AP item because it applies some tanky stats while giving a very good amount of AP. during the building stage of Rod of Ages getting a sheen in the middle of it allows for a large power spike during laning phase. after finishing Rod of Ages getting a morellonomicon gives good CD and AP while applying grievous wounds. Boots of Lucidity also gives a nice amount of CD and the distortion upgrade allows for your summoner spells to be up more often. after getting your core items getting a Lich Bane, Ludens Echo, and a Rabadons Death Cap allow for an intense late game power spike.