Quinn is a very versatile champion. You can build her many different ways and play her in many different lanes (I will make a Quinn jungle guide soon.) This guide will teach you how to play Quinn as an assassin mid.

The build I use for assassin Quinn resembles a classic Zed build, focusing on crits, attack speed and damage. You might be able to swap the Bork for Bloodthirster and the Last Whisper or the Ghost Blade for the Black Cleaver. The last item relies on what your team needs. I'll do a separate chapter on each item that I don't include here. Bloodthirster and Hydra: Good for when you don't want to rely on crits for assassinations and when you need the life steal to survive fights. (Hydra is kind of an experiment, please tell me how you do when you use this item.) Shiv and Phantom Dancer is for when you need to be more of an ADC. Tri Force is never a bad idea. All of its stats benefit Quinn. Good for if you don't know what exactly your team needs. GA and Frozen Mallet are for when you enjoy life and you would like to keep yours. For Boots just look at the notes I wrote on them. I'm too lazy to copy them over to here.

The new Black Cleaver is accually really good for marksman, and not that bad for assassins. The cdr it provides is extremely handy and the health it gives is better than Tri Force. The rage passive is useful for Valor form, while the armor shred is useful for Quinn form. On the other hand, the Tri Force's crits chance and sheen passive can cause you to deal more damage in quick succession. Over all, I prefer the Black Cleaver when the opponents are tanker, and Tri Force when the opponents are squishier.

Phantom Dancer is for when you want to fully be an adc. The attack speed and crit chance is very useful. Shiv on the other hand is for when you want to be more ADC like while still adding some burst. The wave clear is also nice because Quinn lacks the ability to clear waves quickly. I personally prefer Shiv over Phantom Dancer, but both are good items. The crit chance is very big because if you crit with a passive strike is does over a 1000 damage.

Pros

+Very high burst
+Very high mobility
+Great duelist
+Poking is easy with passive
+Vault is a great escape
+Easy to roam
+Easy to farm

Cons

-Being CC'd is almost instant death
-Very squishy
-Weak at lvls 3 through 6
-Long cooldowns
-Both forms share CD's

Laning with Quinn is pretty easy. Farming is easy because of your passive, and it's easy to poke because of your passive. You want to poke the enemy whenever your passive procs on them and follow it up with Vault and another passive shot. If you are facing someone with a channeled spell then save your Vault to stop it. When getting ganked try saving your Vault until the right moment to dodge cc, stop chaneled spells, or jump over walls. If the enemy mid lanes is low when the jungler ganks, then Ult Flash E Ult them to put yourself in a 1v1 with the jungler. Save your Q when jumping on the enemy mid unless it's necessary to finish them off because, you may need the blind to fight the jungler. Quinn is a great duelist so she can so she can easily 1v1 most junglers.

In team fights it is very important that you save your Ult for when the enemies are low and trying to get away, or your team is losing and you have to get away. Don't Vault all over the place either. Save your Vault to reposition yourself, escape, stop chaneled spells, or if your team if the fight is 1 sided and you team is running though theirs finishing off an enemy. In team fights you just need to be another ADC till the need arises to Ult. When in Valor form your main goal is to 1 shot the enemy ADC and maybe kill more, or get out alive. You are completely relient on your team to initiate unless of course you are so fed you can kill 2 of them before they even relies what's going on.

I dont have much to say about these. You can change the masters however you'd like. For the runes you can swap the marks for flat attack damage marks. I only prefer the armor pen because the enemy usually builds armor fairly early on. The rest of the runes are fairly standard.