Try this one out!
AP sion is my main and I'm a fairly humble and honest guy that being said, I win 99% (mostly twisted treeline is where I play him but I'm gonna start trying it on 5v) of my games and carry the team almost every game with AP sion.
Hands down AP Sion is arguably the greatest and easiest and fun to learn.
Masteries: I go 9/0/21 with flash and ignite (see attached)
Runes: I do cooldown, dodge, magic pen or even attack speed wouldn't hurt. (universal pretty much) and EXP QUINTS! (very important)
Sion's death's caress farms creeps like CRAZY! then turn on enrage for the rest of the game (should be able to use wihtout it hurting your HP) you'll build up massive health and with all the extra exp you'll be level 6 while everyone else is lvl 3! I solo against Jax on TT and win everytime! Jax, yi, big DPS can't handle stun and your shield together so you can almost always survive a gank by hitting those and flashing outta there. Farm Lizard, and buff-giving creeps whenever you can. Should be able to solo liz around lvl 5 but i like to do it at 6 and turn on my ulti (it recharges relatively fast). When you get good enough with him always try to solo top lane and get boots of mobility to gank bot and rack up some kills. If in a gank turn on your ulti when teammates are mid/lowish on health (it gives them lifesteal) and will surprise the team and make them greedy for a kill but the lifesteal and attackspeed your team gets along with you dishing out damage and stun will keep them from getting away. It takes a little while but as always the best strategy is being able to adapt him to your type of gameplay. You can be a little greedy with him and turret dive at higher levels by clicking your shield and stunning the bad guy. Shield absorbs a few turret shots, just make sure you can get away.
Skill order: Cryptic Gaze = Q, Death's Carress = W, Enrage = E, Cannibalism = R
W first because it can do a lot of damage to the surroundings (also good for leveling fast and farming creeps) and give you protection to run away, E so you can get your health rising and do extra damage (always have on after lvl 2, it shouldn't hurt your HP). Q last because its EXPENSIVE and by this time you'll have enough mana and items to use it effectively, alot of ppl think this should be first but I've tried it and do not like having it first, I leave myself without mana and have to make a lot of trips back to base = no EXP!
Early game- buy boots first and then stock 2 health potions and a mana potion. Then miki pendent>sapphire crystal> fiendish codex> boots of swiftness or mobility>tear of goddess>glacial shroud>archangels staff>nashors tooth> then go for more AP like Zhonya's ring or spell vamp late game. If you get to level 18 and have these items you'll be unstoppable.
Send me a friend request when you're on and I can show you how I play him I hope this helps.