Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
OK so this is my AP sion build. It has worked quite well for me so far and quite frankly i have not yet run into anyone who i cannot solo lane against and there are very few combos of champs that i cannot handle 1v2. The secret for me is having enough mana regen and building up shield early to make creep waves a joke. If I am laning 1v2 then for the first 2-3 levels I just stay at the back of exp range to make sure you get it all and can't be harassed. If someone is being over-agressive just pop your shield on when you see them make a move past the creeps. Most people who know how to play will turn back cause they know what's coming. when they do that you just wait till your shield is about to blow and get in range of the front line creeps to damage them so it's not a total waste. If they decide to come at you just draw them into tower range and stun them right as they hit the edge, then go blow your shield on them. That will give them 2 shots from the tower your stun damage shield damage and probably a melee hit or two. IF it's a squishy trying to dive then pop ignite on them and watch them run away in vain then die as they get back to their tower. you can farm extremely efficiently this way because you can stay safe and far back just collecting exp untill their creep wave is ready to attack your tower.. then you blow them up in one shot... still getting all the gold you need keeping your exp up and never entering harms way.
I like to wait till i have enough money for my sorc boots and 1 or 2 amplifying tomes before i go back for the first time to shop. When i am ready i just use shield to charge in to the middle of a creep wave and blow them up then dart into the bushes. If you have been playing safe then your opponent will think they are gonna be ganked and should let you push no problem. Depending on how much money you have when you get back i like speed boots and a tome if you don't have enough for sorc shoes. If you have enough for meijas and sorc boots that is perfect, otherwise 1 or 2 tomes in place of meijas depending how much gold you have.(you need the second tome for lichbane anyway so better to use the extra 400 gold if you have it, than to sit on it). At this point you can start to get more agressive as your shield will start getting pretty strong and will absorb decent damage. My ambush strategy is just to controll the bushes. Put your shield up while under cover and let a champ get in stun range. Even a beefy champ who is at full health will be taken down to half health when you sneak out hit them with stun give them 1 melee hit they retreat and blow your shield as you go. The stun lasts 2 seconds which i find is plenty of time to land those and get back to the bush before they have a chance to really touch you. If you think you have them than feel free to go for it. Instead of turning back just stay on them and ignite. You can chase them down and if they cannot flash or ghost away fast enough.. your second stun comes back up very quickly. that is the basic attack combo and works wonderfully. My next item is lichbane which is godly for sion. If you can.. the perfect move is to put your shield up hit them twice blow your shield then stun them as lichbane activation is ready again. It will give you a ton of damage in a very short time and is half magic and half physical. Then for team fighting try to open with that combo and add your ult too that. in team fights i try to leave shield up as long as possible as people tend to focus AP sion. One great combo for sion in a team fight, is to stun their dps and hit your ult hit them once then put up your shield. The trick with lichbane is that it scales off AP but adds to your physical damage, so if you have 300 AP and 100 AD then your next attack will be 400 damage. Thsi means that when you hit your ult, your next attack will have 400 damage and the regen you gain from your ult is based off that. then you can stun someone and get it again and put shield up and get it again. This also works when attacking towers. If you hit your ult you gain attack speed and lichbane procs, then stun a creep if there are any nearby to let it proc again, and finally hit your shield for a third proc. Depending on how many items you have and how well your meijas is fed i find i usually have about 600 AP endgame and about 160AD. So when i go to attack a tower i can use those three spells and hit the tower at about 760 effective attack. Now towers have a lot of defense but even with that you can still do 100 damage regular hits and your lichbane hits will do about 500. With this, i can BD a full tower in 2 runs and take minimal damage (most is absorbed by the shield). Well that's my sion hope you like it