Sion has a deceptively powerful laning phase for a tank. So be careful of his Q poke. Try dodging it by moving left or right instead of walking backwards.
Post 6, Sion has good gank set up and can escape ganks quite easily. His kill pressure also increases, so be mindful of his level 6 power spike.
Extended trades do not work in Sion’s favour. If you can bait out an ability, you could look to trade and abuse his cooldowns to get him low.
Sion can surprise targets that are walking around Summoners Rift alone by waiting in a bush and using his Q to take them down. Avoid walking in areas where you know Sion has been so its harder for you to get caught out by him.
Kite and disengage is Sion’s biggest threat in a team fight. Try to keep him away from your team so he is unable to get a good Q knockup.
Avoid grouping and fighting in the jungle as it will allow Sion to either get a really good knockup with his Q or get a really good Ultimate R off. Group together, but do not stand on top of one another so he is unable to get a multi-person knockup.
One of Sion’s power spikes is at level 6 when he unlocks his Ultimate R. It offers him extra kill pressure and survivability.
Sion’s item power spikes depend on which build he’s going. He has two distinct build paths (damage and tank) and each spike at different times.
Less lane dominant champions should give him some space at level 3 as Sion can do a surprising amount of damage in the right matchup.