At lvl 1 take Boots of Speed and 3 HP pots.After that take 2 Dorans Rings.Then Hextech Revolver,and run into Will Of The Ancients(u must be thinking that im mad couse i play cassi with spellwamp but pls belive me and try)!Take Needlesly Large Rod and rush for Rabadons DeathCap.Take Catalyst the Protector and go for Rod of Ages.Void Staff will give u good Magic pen. so rush for it.And last item is Zhonyas Hourglass,but if u want u can take if insted of Rabadons for more deff game but u wont need it :D
GL and HF with Cassi :P