I tried all the AP sion guildes out there and they all fail late game, if you get past around 40 min mark they fall apart, early game they are amazing, but if you are going ap sion, let everyone know you are doing so, thus a real tank can step up and tank.
The shield is pre armor reduction, Takes little to no damage to remove the shield from you, a quick cast and its gone. Your better off getting more armor and just voiding off the damage all together.
Also a quick stun and your down for good.
This is a typical situation that happens a bunch, you shield, go to stun -banshie voids the attack stun damage, and your hit with ashe arrow and do ZERO damage and die saying well that sucked.
Also note that even if you do 900 damage with your shield you lose 50% of that damage, so only 450 damage. Now an AP build is good for mid to low ELO games but higher up games and it starts to fall apart, when people run anti-CC builds and focus CC. All it takes is for their primary target to have a stun and banshiee and your whole damage goes out the window, and all you become is a high hp ryze. Not that it's a bad thing but does have problems later on.
I find a solid DPS build works but is slow to build, have to be patient, and farm your gear. Need to work with a solid team as well. Key is for them to realize your team lifeleech abilities, and stay with you.
A solid DPS sion build does around 300 base damage and about 600 crit damage. So you shield, "sheen for 80% more damage on stun, then just crush them for 600+ more damage and finish with shield blast.
So both ways AP and DPS work, key is to fill in the missing role your team needs. And or offset the opponent. IE if an olaf is in play, remember 1 of his lightings will void your shield off, and he can't be stunned. He will put you out of play the whole game.
If they are heavy tank builds then doing 3k damage aka about 1500 after resists, will just annoy them and you have nothing else to offer.
If they build a bunch of casters that are squishy or there is a soraka in play on their team, then you can use an AP build to just ruin their day.
So learn to pick which build works best and go with it.