Nasus vs Kennen

Last-hit minions with Siphoning Strike (Q): each kill with the spell permanently increases its damage, by more on large minions and champions.

In teamfights, cast Wither (W) on their carry: the slow grows stronger over the spell's duration and also reduces their attack speed.

Cast Fury of the Sands (R) as you enter a teamfight: for 15 seconds, you gain health and resistances, damage nearby enemies, and Siphoning Strike (Q) comes back faster.

Early on, farm rather than force trades: Siphoning Strike (Q) stacks are permanent, and every minion you last-hit makes you stronger later.

Dodge Thundering Shuriken (Q): each spell that hits you applies Mark of the Storm (passive), and on the third, you're stunned.

Back off as soon as he rushes at you with Lightning Rush (E): it's his engage, often followed by Slicing Maelstrom (R).

When he has Slicing Maelstrom (R), spread out from your allies: the storm hits every enemy around him, and its repeated hits end up stunning.

Wait for Slicing Maelstrom (R) to end before you fight back: during the storm, he gains armor and magic resist, then the spell has a 120-second cooldown.