The problem is all they've ever done is nerf him. His kit is broken, not his damage. Both his main damage spells have less than a second cooldown and no mana restraints to regulate spamming. His damage has 5 variable multiplication (AD, crit chance, crit chance doubling, crit damage multipliers, and built in armor penetration on ult). He has 3 different defensive pieces (shield, insane mobility kiting, and projectile blocking). Instead of a normal passive, he gets 2 which are each better than a normal passive.
There's a number of things you would have to do to fix him. Move his crit chance increase and damage decrease to scale on level with his windwall so he has to max that first to get double crit chance. Flat out remove the armor penetration on ulting; he has way too much built in multiplicative damage already. Put a hard CD (maybe 6 seconds) with a visible effect on his E after he uses it and then goes 2 seconds without recasting it. Make his Q's CD scale on level instead of attack speed.