At level 1 you must try to get as much poke on your laner as you now can, because when you hit level 2 you will go all in, to get the first blood, because you have your Q-E combo, and the ignite aswell, i would prefer as backing around 850G and buy a HP pot, a sight ward, and the tear of goddes, and then as fast as you know can you will pack, and start building your voidstaff, as soon as you have 750 mana, you will upgrade your tear... And in the meantime, you need to have your boots fully upgraded, with the upgrade also, for roaming and stuff like that. I'd prefer in early game swapping the trinket, to a Orb, or a sweeper(depends on the elo aswell) High elo, i would prefer sweeper, and in low elo i'd prefer laser, since people usually wards their lane bush, with the trinket.

Mid game, will the cassio get very aggesive and strong, i'd prefer roaming as much as you now can even tho you're behind or ahead. If you lose mid badly as Cassio, i'd prefer roaming as much as you know can, to keep up.. If all lanes are losing, i'd be farming till you get the tar fully stacked, and the voidstaff, even tho void is a late game item. I would highly recommend it to the early pressure.