In the laning phase you should focus most on CS, only trade when they walk up for CS.
If you are playing against a ranged champion you should stay far back and take CS with your Q.
Always try to get the kill once your jungler comes and ganks you.
When playing against a ranged champ 1 kill in the early game is massive, this makes you WAY stronger then them and you no longer need to farm with Q.

To trade with Lee in the mid lane you should: AA, E and E2 this will proc electrecute and will do a lot of damage. You can also add in Q and Q2 if you are certain they will not do more damage then you. ONLY TRADE ONCE THEY STEP UP FOR A MINION, NEVER FORCE A TRADE

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The new mythic Evenshroud gives you alot of takiness while still being able to one shot almost anybody, also helps out your team a lot.

The new legendary item Axiom Arc is great for Lee sin because it gives you lethality and reduces your ultimate cooldown.

If you wanna go more of a bruiser build you can go with goredrinker and conqueror runes. Personally I dont think Lee mid is a good bruiser but you can test it out and see what you think.

If you wanna go more of an assasin build you should you go with prowlers claw, it works great with Lee since it puts you behind the enemy which means you can kick them right into your team.