Lux vs Nasus

Start with Light Binding (Q), then Lucent Singularity (E) and Final Spark (R) before detonating the singularity: the root makes sure everything hits.

Slip in an auto after each spell that hits: it triggers Illumination (passive) for extra magic damage, and Final Spark (R) triggers it too.

Cast Lucent Singularity (E) into a brush before walking in: the area is revealed, and a hidden enemy would be slowed.

Cast Prismatic Barrier (W) through as many allies as you can: the wand shields them on the way out, then again on the way back.

Cast Final Spark (R) to clear a wave or secure an objective: it only has a 60 to 40 second cooldown, depending on rank.

Early on, take a trade whenever he steps up to last-hit a minion: each kill with Siphoning Strike (Q) increases its damage for the whole game.

Freeze the wave near your tower: to farm with Siphoning Strike (Q), he then has to step up and expose himself to ganks.

Once he hits level 6, don't engage during Fury of the Sands (R): for 15 seconds, he gains health and resistances, and his area damage scales with your max health.

In teamfights, if you're ranged, hit him while backing off: he has no dash, only Wither (W), a slow that grows stronger over time.