Cassiopeia has a powerful level 2 all in with her Q and E spam. Make sure you hit level 2 at the same time which is the first wave followed by 1 melee minion on the second wave to reduce her ability to kill you.
Cassiopeia has mana issues in the early game. If she is low on mana, it could be a good time to fight her as her damage output will be heavily reduced as she relies heavily on mana to trade.
She is an immobile Mage with no escapes. Let her push the wave and then call for assistance from your Jungler to shut her down.
In team fights, do not group closely together to make it impossible for her to land a 5 person Ultimate R. Be prepared to turn around if she looks like she’s about to cast her Ultimate R.
Never fight the enemy in the jungle or around an objective as her Ultimate R is really good in tight areas when teams are clumped together. Fight spaced out in a lane when possible.
Cassiopeia will look to ambush or assassinate targets that are moving through the river alone. Unless you know where Cassiopeia is, do not walk through the river or around the map on your own especially if you’re squishy. If you die, she may cost your team the next team fight or objective.
Cassiopeia has a lot of mana problems in the early game, but once she gets Tear of the Goddess she will play a lot more aggressive.
Cassiopeia really shines in the late game. She scales better than most champs and she can build 6 items. If you get ahead early- try closing out the game as soon as possible.
Not many champions are as strong as Cassiopeia is at level 2. Beware of the level 2 all-in with her Q and E spam.