I just found he can 1v3 and get ace with his ulti.
Not sure if focus him or not in teamfight as he's tanky with his shield and high health, but doing ton of damage.
He can get kill easily in level1 teamfight with his stun.

How to deal with AD and AP sion?

ignite, executioner's calling, or use a stun and focus him to death

AD sion is hard countered by cc his survivabilty is low if not attacking as it comes from LS

AP sion is trickier play safe and ward. If u don't get ganked pre6 your ap should be stronger 6 on. Just be careful and know his survivability/dmg is very high early game.

AD sion is only a beast if you let him farm his e. But if you're stupid enough to let him farm and not target him early good luck trying to get your win on.

Try to bait him into using his ult early, then make a full retreat, wait 20 seconds, then engage again.

I find Riven to be pretty strong vs Sion because she has much better mobility and some CC to interrupt his auto attacks. Also, exhaust and ignite.

ignite exhaust and burst

Amumu

plenty of CC to stop him from ulting/auto attacking...
if he does get his ult off... kite and run till it goes down.. it is very long so care. Towers will help you greatly since the average player doesn't gear to much def
a /surrender always end the game and puts you in the clear lol
ignite is only so good... if he does have def and plenty of health then it might not be long enough to keep him from lifesteal... exhaust can also help since lifesteal is based on how much damage is delt so team cordination is key in keeping is damage at a minimum as to not blow all you summoner skills at the start of the fight and to stager them

Dark Wolf knows whats up

Ignite, Grievous Wound, Exhaust, CC, Damage. GG Sion.