Stand away from the minion wave when possible. This will make it harder for her to poke and harass you with both her Q and W.
Don't give Sivir free mana by randomly poking her with abilities. Only use your abilities to poke with if you’re in a position to go in and kill her.
Post level 6, avoid overextending when low as Sivir’s Ultimate R can be used aggressively to catch you out while you’re overextended.
Sivir will be left with her Support in the mid lane at times. If your team is in a position to do so, you could collapse on her and the Support and then force an objective like the Dragon or Baron with the numbers advantage.
Group as a team and avoid splitting up too much. Sivir’s Ultimate R is a great tool to initiate team fights and if she’s able to collapse on a target who’s alone or overextended.
In team fights, make sure you can bait out or play around Sivir’s Spell Shield E. If you’re able to play around it, killing her will be soo much easier.
Sivir is very strong once she has tons of crit. This makes her pretty strong during the mid and late game.
Once Sivir unlocks her Ultimate R, her kill pressure increase but not in a usual 2v2 fight. As it’s a utility tool, it allows her to chase you down or hard engage with her Jungler. Expect her to get ganks post 6.
Sivir is really good in team fights as her Ultimate R can help her team catch out enemies who are overextended or out of position. Avoid splitting up or overextending in the later stages of the game.