Top Cassio is strongest when farming itself becomes unsafe: keep the wave where the opponent's last-hit step is readable, because one forced walk-up in a long lane can turn a routine CS into a full poison chase
Place W on the line they need to use, not just on their model, since Miasma is what makes many top-lane melees fail to leave cleanly and sets up the easier Q that keeps the run-down going
In fights, do not wait for the perfect Flash-R carry clip; turning ult on the people already coming in and chewing through the bruiser you can safely keep poisoned is often the clean Cassiopeia carry pattern
When you fall behind, stop trying to recreate the lane with one forced trade. Your job becomes making their first commit awkward enough that they have to spend extra time and cooldowns to finish anything. Keep the wave in a spot where you can still punish predictable walk-ups, save W for the path that actually lets them stick to you, and only step up hard after a clean Q hit. In fights, drop the idea that you must reach their backline; poison the first target you can keep in range and keep casting while backing through your own space.
All Move Speed bonuses are more effective on Cassiopeia.
Cassiopeia blasts an area with Poison after a brief delay, granting her increased Move Speed if she hits an enemy champion.
Cassiopeia releases several clouds of poison, slowing, grounding, and lightly damaging enemies that pass through them. Grounded enemies cannot use Movement abilities.
Cassiopeia lets loose an attack that deals increased damage to Poisoned targets and heals her for a percentage of the damage dealt. If the target dies from this attack, Cassiopeia regains Mana.
Cassiopeia releases a swirl of magical energy from her eyes, stunning any enemies in front of her that are facing her and slowing any others with their back turned.