It is not that AP Varus is too strong. Unlike Kaisa he has to basic attack you 3 times before he can burst you, or hit his ult (which is not that diffecult to avoid) follow up an ability.
----------------Short pause to point out how the death recap fails once again in this picture----------------------------------------------------------
What the issue vs AP Varus is, is that itemization against AP is bullshit.
As Renetkon with Asaptive Helmet, Spirit Vsage and lvl 18 you will have 163.4 MR, which would be 62% damage reduction.
Varus has void staff, so only so only 65% of your mr counts, which equals 106.21, so 52% damage reduction.
Varus has at least 538 AP, so his w passive dealt 15% of our max health per stack.
Now in this scenario Varus DID hit his ult and most likely triggered 6 of these w stacks, so he dealt 90% max-health magic damage.
If we reduce that by the what you normally stack on mr that I already calculated the reduction on. It is still 43.2% of your max health.
And that is not adding up the damage dealth by nashors or his w on hit damage or the Q or the E.