Kalista and Thresh are a very dangerous duo in the bot lane, with an extremely strong early game and a ton of engage tools.
Kalista has a very unique basic attack, as she dashes every time she lands one. While this makes her incredibly mobile, her basic attacks are uncancellable so she is vulnerable while her attacks are winding up. However, attack speed reduces the duration of her attack windup so she becomes more agile as she scales into the late game.
Most of Kalista's damage comes from racking up spears into her target with her Q and basic attacks. Once a good amount of spears have been accumulated, she can pop her E to deal a huge amount of burst damage depending on the amount of spears the target has. This makes Kalista fantastic at securing Baron and Dragon.
Meanwhile, Thresh is a great pick throughout the game. During the laning phase, his hook serves as a perpetual threat to the enemy and can be comboed with his E for a lethal CC chain. After you've landed a hook, it's important to wait until the stun is about to expire before using E to pull the target back into you; this ensures you maximise the duration of your CC. E can also be used to push enemies away to disengage.
At level 6, Thresh's ultimate serves as another form of CC. The Box can slow enemies after they've been hooked, or be used to disengage from enemies by zoning them away. Enemies can also be dragged into The Box with his E.
Every time Thresh lands a hook in the laning phase, this serves as a fantastic opportunity to Kalista to turn onto the target and start ramping up her spears. Once the enemy is close to getting away, she can use her E to deal a huge amount of damage. This is a very consistent combo throughout the laning phase and often leads to a kill.
As mentioned earlier, Kalista is vulnerable while her attacks her ramping up. Thresh's W is a fantastic way to help Kalista escape whenever she finds herself in these tricky situations.
Once Kalista has unlocked her ultimate, this serves as another engage tool for the duo. Thresh can be launched into the enemies to knock them up, before using his E to pull them toward his allies or trap them in with his own ultimate.
Runes and Items
Kalista's runes and items are focused on attack speed, as this reduces her basic attack windup and increases the amount of spears you can accumulate on a target. Immortal Shieldbow makes Kalista more durable, while Runaan's Hurricane allows her to add spears to multiple targets with one basic attack.
Thresh's keystone rune is pretty versatile; Aftershock makes him tankier while engaging, while Guardian provides a solid shield for you and your allies. Hexflash can be used from a brush to catch enemies off guard with your hook. Locket of the Iron Solari is a great mythic by making Thresh and his allies tankier, while Knight's Vow or Redemption can be used to provide further protection for your team.
Thresh and Kalista are a consistently strong duo across multiple metas. They have a very oppressive and dangerous laning phase, and transition into the mid/late game with great pick potential.