Here are my tips when going against Ryze.
Ryze can burst down targets quickly, so getting health items are a good buy. Mercury Treads is also an excellent item, it not only gives you MS (for getting away from Ryze), MR (Ryze does mainly magic damage), but also Tenacity (permasnare gone)
Use the fact that Ryze's abilities are point and click to his disadvantage. Remember that he cannot target units in unwarded bushes. Saying Ryze is overpowered is like saying Taric is overpowered with a point and click stun.
Overload is a skill shot, but it doesn't do much damage. Don't get into a bad position just to dodge it. For example don't stand next to your minions so that they block Overload. Spell Flux does exceptional damage to targets that do.
The best time to attack Ryze is not when he has less than 3 stacks. That is wrong. Ryze only needs 1 stack. The best time to attack is RIGHT after Arcane Mastery wears off, not when he's building it up.
Ryze is weak when his passive is off, powerful when it is activated. Use the stacks around him to your advantage. He's like Gnar; when his passive is up, he becomes Mega Gnar. Use this knowledge on Ryze to defeat him please :)