Nami will walk up to you and use her W on herself (or on you) in attempts to poke and make the wave bounce. Do not stay in range of her so she cannot get health back via the bounce.
When you poke Nami, do not walk directly backwards as she can easily Bubble Q you. Instead, try and walk to the side.
Look to trade with Nami when her W is on cooldown as her damage output will plummet.
Invest in an Executioner’s Calling to reduce Nami’s healing on herself and her allies. This item is invaluable when the enemy has lots of sustain.
Taking down Nami first before a team fight occurs will make it really difficult for her ADC to survive because they will not have any peel or protection.
Avoid team fighting around objectives or inside the jungle as Nami’s Ultimate R is rather large and if you fight in tight areas, she’s guaranteed to land multiple knock-ups.
Nami’s poke at level 1 and 2 is pretty deadly. Don’t let her bully you down at level 1 so it’s harder for her to look for the level 2 all-in.
Once Nami hits level 6, her kill pressure intensifies as her Ultimate R is a great tool to start skirmishes or disengage from fights. Make sure you have vision in your lane as she may use her Ultimate R to set up her Jungler.
Nami is very good when she has 1 or more core healing items as she will be able to sustain and heal her allies back to full health quickly. Try to put her behind in the early game to deny these power spikes.