It may be common sense to some but not all...
What happens when Berserker (without any equipment, in this example) uses his skill and it successfully hits an enemy hero that has a Berserker aid on it?
You have to see the hero aid and skill as 2 different functions. I'd like to refer you to This Thread for a more detailed explanation of The Berserker aid
His main skill lets the enemy hero take 2 times more dmg at most for 12 seconds. This means that your target is going to have only half the hp.
However, if the enemy hero has Berserker aid, it will mean that after a set dmg, the hero will no longer take dmg. See the thread to see what I'm talking about.
I hope this will be helpful information to you.