Extremely good point.
Basically, what counters %health true damage is healing/shielding after damage is done, or ways to stop the champion from attacking you.
If you heal with any of these items (Potions (
, Relic Shield line (
), Ancient Coin line (
), Regeneration items (
), Valor's Reward (
,
), Mikael's Crucible (
), Overlord's Bloodmail (
), Lifesteal/Spell Vamprism (
), you are prolonging your life much more than if you had flat health. For example, the Ruby Crystal gives you 150 health and a Health Potion regenerates 150 Health, but if you pop a Health Potion at the beginning of a fight against a %health true damage user, you will survive longer than the person with a Ruby Crystal.
I believe that if you have a shield at the beginning of the fight, you will not prolong your life, but if you shield just as you are about to die, then you will live much longer. So
and
have a chance to prolong your life, with a higher overall boost to survivability the less life you have. (So in this regard, I don't believe
or
help you.)
Also, lastly,
is another counter, being able to heal 33% of your life after you die.
So that's healing and shielding.
What you can also do is stop the enemy from attacking. This can come from attack speed slows (
), invulnerability (
), or outmaneuvering the champion (all those boots and movespeed items). (The enemy can't attack you if you're not in range).
Lastly, damage. If tankiness isn't going to help you as much as you'd like, then just try to kill the %hp true damage user before he or she kills you.
"Death is the best cc."
So basically, you can counterbuild %hp true damage. It's difficult, but possible.
This is also exactly what it's meant to be - difficult. It's not supposed to be something that you're trying to counterbuild. You don't counterbuild to fight the true damage, you counterbuild to fight other aspects of the champion, and if you don't want to/can't do that, then you build to your champion and to your allies. %HP true damage is in the game because there need to be some champions that hard-counter tanks. Because of its nature, it is also effective against lower-health targets. That is okay. Not everything needs a counter. As long as it is balanced, it can exist in the game.
For example, Guardian Angel is has an uncounterable passive. You can stun a person who has Zhonya's Hourglass and kill him before he or she is unstuck, or you can kill the person so quickly that the person has no time to react. These methods are inapplicable to Guardian Angel - the person will respawn, and will have a chance to get out alive.
However, despite being uncounterable, Guardian Angel is a balanced item.
Same with the concept of %Health True Damage.
(I edited in Sterak's Gage and Seraph's Embrace as shielding items after I saw a comment from Quinn of Yuri. Though that comment wasn't meant specifically for me, thanks! I forgot about those.)