Which core heroes are protected in the backline? Anyways you should always just focus the enemy backline as lesley, the tanky heroes might be stung by the true damage but the squishies are extremely vulnerable getting 2 shot in less than 2 seconds if done correctly. The squishies usually have atleast 4500-5000 HP which is enough for many lesley builds to 2 shot.

Also if the tanky fighters start pushing into your team in a teamfight just attack them and run when you're being pushed back, If done correctly you can kill them easily and maybe do a counter attack.

If CC heroes like chou, atlas, etc. Are running towards you immediately run and retreat and don't look back until they use their CC. Some CC heroes have flicker combos you need to be aware of.

Also if you need tips to play well for the early game, just build a variation of warrior boots/tough boots + iron leggings/magic blade you build warrior boots + iron leggings if the enemy is physical damage oriented and tough boots + magic blade if the enemy is magic oriented.

Building these items will give you tons of physical resistance and magic resistance which is crucial to surviving the early game, if the enemy ganks your lane 3v1 just run and leave your tower.

Don't poke your lane counterpart until you get berserker's fury. Your build should be rapid boots, berserker's fury, blade of despair, endless battle, 2nd blade of despair, and then hunter strike with mm emblem with 3 points into attack and critical damage with the 1st mm talent that gives 15% extra physical scaling.

If you build the early defensive combos stated beforehand you can sell those items in the late game and build rapid boots instead. Positioning is key with lesley and always communicate with your team if you need some help or if you want to set up an early gank.