This way to play Sion is the best I can imagine!
Here he makes huge crit dmg around himselfe and healing all mates around himselfe for the
half dmg he do!This way sion is a BIG CARRY so at the beginning he's very useless early teamfight you'll lose but when Sion get his first Tiamat you'll win.

I take the runes with crit dmg cause all other runes don't make more sence then these.
With all items he have 100% crit and with crit dmg runes he do about 900 dmg.

I take the normal carry-crit dmg masterys to run behind an enemy and leveling fast and do huge dmg.But a bit Ap for the shield

I take tiamat because sion is healing him and his mate for all dmg he do while his ulti and he do around him the same dmg like on the target cause tiamat's passiv is no unique-passiv so it's stackable.If Sion do 500 dmg and 5 targets would be around him he would heal himselfe for 2500 and his mates for 1250 EACH HIT and with phantom dancer and his ulti he has a good attackspeed.

Exhaused for slowing movement and attakspeed from enemys and ghost running after them and waiting for stun couldown^^

With this build sion do a huge aoe dmg and healing aoe with his ulti.Nobody can run away from him when he using ghost his stun and exhaused.He healing himselfe and his mates so fast.
But there's one way to kill him full eq ... 5 secs stun xP

At the beginning stun your enemys and play def.You've to wait for your Infinty Edge,but later with lvl 11-18 sell your mana crystal it's only for the beginning for the huge manacosts for the Q-spell.Then the general skill use use Q for stun and W for your shield and a bit dmg.You can blok with your shield ennoying ultis like karthus or fighting at tower,wait till attacked by enemys and then stun shield and attack.