I made the lethality Sivir mid guide back in like November since I thought it was a niche/different way to play her, while also doing a lot of damage. Fast forward to now, it's wild that lethality is sort of becoming meta for her. Glad it has gained popularity!

Extra side note: I think Eclipse is a viable mythic too due to the armor pen passive, however Duskblade's ability haste passive feels really good for Q spam.

I originally made this guide for mid, but you can definitely play it bot. In general, your goal should be to land as many of your Qs as possible to delete enemies. For this Sivir build, your Q is massive for your overall damage output, rather than traditional Sivir builds that rely on her W for heavy teamfight crit. damage.

In lane, try to not Q into a wave, as it decreases its damage. Look for situations where you will only hit 1-2 minions on the way through (ideal situations obviously being a direct hit for max damage). When facing melee champions, look for lots of opportunities to poke with your auto attacks when they approach the wave to cs. Particularly if you know they aren't willing to give up a cs (say a cannon minion), this would be a great opportunity to get a huge Q chunk when they step in front of their wave to kill the cannon minion. When you know you are likely to get a free Q on them, try to distance yourself so that you hit them at max range, ensuring that your boomerang hits them twice without hitting caster minions behind them on its way back. This will maximize its damage. Use your E to restore mana. You will be decently mana hungry, but don't be afraid to use your abilities to punish enemies. You will probably have the wave shoved to their turret a lot, so go for free auto attack and Q poke when they are trying to cs under turret (Be sure you are safe from enemy ganks be having vision around lane, or at least having your jungler close by). Use your E on easily telegraphed abilities like Kassadin or Ahri Q for easy mana regen. Save E in more dangerous situations for hard cc/burst all-in attempts against you. Prioritize blocking hard cc. You really should be able to have good cs in the laning phase. Even if the enemy midlaner is winning lane and has kill pressure, you can elect for a more passive laning phase and just farm safely under tower.

With this build, late game your Q will have a relatively low cooldown, so you will be able to throw a lot of them. Try to pick off enemies. If a bruiser/assassin is going after you in the back line, use your E for blocking cc and movement speed for quickly peeling off and reengaging after. You can also use your ult in the same way, if the situation is more dire.

Remember that even though the beauty of this build is the damage chunks from your Q pokes, you still have auto attacks. You are going to be building a lot of ad so don't be afraid to be autoing a lot in teamfights. That is how you will be pumping out your optimal amount of damage. Whether it be your Q, autos, or your W (don't forget you have that), you will hurt.

Technically a major flaw of this build is that you should be very mana hungry throughout the game. I actually don't find this to be that much of an issue, despite the big mana costs on her abilities. Whether it's because of stacking your manaflow, or your tear, I really don't think mana is too problematic. You may not want to take long drawn-out teamfights though. Of course, all of this is of very much less consequence if your team is nice and spares you a blue buff every now and then :).

Truthfully, I haven't played this Sivir build enough to know what the ideal team compositions she fits into are. You should be able to play this, even with a traditional ad carry, since the lethality you are build should negate the armor that squishier enemy champions might build against your composition. That being said, perhaps this build would be more ideal with ap bot. Either way, just make sure your team comp has enough ap somewhere so that the enemy team can't fully itemize against you.

Additionally, I am not really sure who poses the biggest threat to you in lane. Someone like a Zed might be dangerous since I don't know how Sivir's E interacts with his ult. Fizz might also be bad, especially if you do not have your E up to block his ult. Sett and Rumble could also be difficult, since they could gap close you and then you would be in trouble. At heart, you are still a traditional, immobile ad carry, so you want to be keeping your distance in lane (and in teamfights) at all cost. Essentially play this build like you are playing ad carry, though Sivir's E allows you a little more freedom in lane (and in teamfights).

At adc, on hit burst champions can be really dangerous against you in lane. I noted tristana and lucian but there are others as well. You just need to play safe in lane against them and you will be fine.

Synergy wise, same thing as when you play and other ad carries. It is nice to have champs that can peel for you in teamfights. Your ult is also really good for reengages for your team, or for disengages from a potential losing fight. Of course, using it for straight up engages is always a possibility too. Her ult is flexible, and it is no different for this build.

A little note about my side laning thoughts: I think because of her mobility in this build (passive, ult if need be) she could be somewhat safe on the sidelane pushes. She won't be underleveled like traditional ad carries because this is midlane Sivir. I'm not saying I would take aggressive all in fights if your split push is matched, I'm just saying you should be confident in your ability to shove a side lane wave and escape if forced into a risky situation with all your movement speed. If you determine that you want to play in the side lanes late, run teleport to quickly rejoin teamfights (like any other split push focused champion does).

That is all I have for you. Good luck out there. You can do it. I believe in you (and this build).