Honestly when I first tried this out I was bored. But needless to say it worked out better than I expected, so here's my "know-how".

Nami Top is a pick ( not recommended for ranked on higher divisions, duh! ) but really fun and worth a try. So far i've managed a Quadra-kill with her combos and no doubt, she could possibly do a Penta as well. But you're probably asking, what is so great about picking Nami over other top champs? Well, not only does she dominate her lane against pretty much most top champs(Galactic Poke with her E turns into superb damage), but her endgame does so much damage you don't even need a tank - that is if your team didn't feed all the enemies.

Pros
- She can 1v1 any carry with her Q -> E -> Auto Attack Combo
- She can destroy the whole team if she lands her complete Combo Sequence and the team is in a narrow spot together, of course you should have your RH for that spread damage.
- She's fun
- She has lots of cc, from a slow to not let the enemy get away to her bubble and AOE ult.
- She has decent sustain thanks to her W (Potions, lol wut?)
- People will usually not focus you in a TF until they realize you killed them and are a critical bastard fish. It's kind of like seeing Blitzcrank in a fight, you're used to not going after him.

Cons:
- Her bubbles are seriously hard to hit, so missing her combo might equal death.
- Most of her abilities require some sort of skill to use consistently well and you must have great positioning at all times.
- Weak against high mobility champs *A.K.A. Le Blanc, Akali and so on..*
- Fairly squishy depending on build

What are the best ways to one-shot other champs?

Personally the best combinations i've come up with so far are:

Your Poke: E + AA ( For slow and kiting... it hurts.. a lot. )

Your basic engage combo: E + Q + AA ( That stun gives you 1.5 seconds of free damage) and since you stacked Crit and Attack speed .... 1 Basic engage is almost instant death.

Your Complete Combo: R + E + Q + AA

Why start with R? It's simply easier to land your bubble afterwards. If you miss your Ult, it's a sign for you to back off, specially if going against a tank. Late game you don't really care as much since landing your bubble will be enough to kill or double kill if 2 people get hit by the bubble - that plus your Runaan's Hurricane seal the deal.