Void staff isn't nearly as broken as you think
Imagine you have 200 magic resist. That leads to a 66.6% reduction in magic damage.
The enemy has a void staff and masteries, resulting in 39% Magic Pen + 22 Flat. Round that to 40%+22.
200*.6 = 120, 120 - 22 = 98 magic resist. The final result there is 50.5% reduction in magic damage.
With void staff, masteries, and boots, they only end up shaving off 16.6% off of your reduction. That isn't much. At all.
1500 damage with 66.6% reduction: 500 1500 damage with 50.5% reduction: 757
As you can see, one only does 50% more damage than the other. That isn't much at all, just build 250 health to make up for it. Legit easy.