Not Updated For Current Season
Early Game: try to haras you enemy as much as you can with null sphere. At lv 5 go to kill enemy to lv up till 6 lvl and do teleport to shop.
Mid Game]: try to go in enemy jungle and when its time do riftwalk to enemy and finish him with null sphere and focus pulse.
Late Game help you team in team battle but just take position near battle not in. Kassidin is not a tank you cant go directly in battle and you can chase enemy down with riftwalk