Don't be afraid to constantly cast your abilities as Death Rate Death Rate makes each additional use cheaper. Noxious Blast Noxious Blast doesn't require a target, so maintaining all 5 stacks is easy with a decent amount of mana.
Try to get all 5 stacks with Deadly Cadence Deadly Cadence before casting Petrifying Gaze Petrifying Gaze to reduce its cost significantly.
Cassiopeia Cassiopeia is a great stalker throughout the game. His three basic abilities, especially Noxious Blast Noxious Blast, can be used constantly to damage online, as he is capable of dealing large amounts of damage per second at a safe distance.
Harmful Explosion Harmful Explosion has a small delay; Follow your enemy while targeting Noxious Blast Noxious Blast to secure the hit, or use it when they are stationary or about to finish off a minion.
Using Noxious Blast Noxious Blast on a bush doesn't reveal Cassiopeia Cassiopeia, so take advantage of the bushes on the top and bottom lines to attack without taking damage from minions.
When escaping or chasing, be sure to use Noxious Blast Noxious Blast on an enemy champion to improve their speed and Miasma Miasma to slow them down.
Using Miasma Miasma in the middle of the lane when enemy minions are coming slowed them down and will do a little damage to them so that your own minions have an advantage.
Since Miasma Miasma gives vision over the affected area, it can be used to tell what there is of a bush. The Noxious Blast Noxious Blast can also be used to make sure if someone is in a bush, as it gives you extra movement speed (and an upgrade icon) if an enemy is hit with the ability. If you threw it on a bush and you get the speed buff, you know the enemy is there.
Twin Fangs Twin Fangs become extremely powerful if the target is poisoned, allowing a large amount of damage to be dealt from its short cooldown.
It is highly "recommended" to put Twin Fangs Twin Fangs in Quick Spell since it allows to cast more easily a greater amount of Twin Fangs Twin Fangs in the duration of the poison.
If the situation is favorable, try casting Twin Fangs Twin Fangs being very close to your target, as Twin Fangs's Twin Fangs's cooldown resets when the target is hit, not when they are thrown.
As Twin Fangs Twin Fangs have the potential to be constantly refreshed, don't be afraid to target a melee champion if necessary. Unlike many mages that require explosive damage, Cassiopeia Cassiopeia and her Twin Fangs Twin Fangs can destroy enemies with time, aim, and enough mana. Twin Fangs Twin Fangs have their cooldown reduced to 0.5 seconds if they hit a poisoned enemy, regardless of the source of the poison. So having a teammate like Teemo Teemo or Singed Singed is very helpful for Cassiopeia Cassiopeia.
Attempts to use Petrifying Gaze Petrifying Gaze offensively and usually against more than one targets. Also, it can be used when ambushed to escape from enemies.
However, it is safest to use it during a team fight. The enemy team will be less likely to turn around to avoid the effect by being focused on 5 champions and possibly they are looking towards you to face your team.
Using Petrifying Gaze Petrifying Gaze followed by Miasma Miasma, Noxious Blast Noxious Blast, and Twin Fangs Twin Fangs can turn 1v1 fights and tower ambushes in midgame to your advantage.
The Petrifying Gaze Petrifying Gaze also allows Cassiopeia Cassiopeia to be a great character in the face of ambushes. Simply throw it on your online opponent and their jungle and kill one of them quickly, leaving you alone against 1 enemy. This is most effective when Petrifying Gaze Petrifying Gaze stunned both targets.
Cassiopeia Cassiopeia Can farm proxy using Noxious Blast Noxious Blast and Miasma Miasma.