Here's how to defeat or counter Naix:
Disables are Naix's biggest fear. Rage only lasts less than 5 seconds and as long as Naix's stunned, he can't deal any damage, regenerate or Infest, leaving him vulnerable on the spot.
Focus Fire with your team to bring down Naix in team battles. Even if he has Lifesteal and Feast, constant damaging, especially from DPS heroes, will make Naix's health drop rapidly.
Slow will destroy Naix's killing potential. Since Naix is melee, if you slow him down every time he tries to kill you, there's no possibility that he'll catch up unless he's with an ally.
Call for help will save your life if Naix is in the process of killing you. If Open Wounds did not kill you, chances are if your teammates arrive in time, they can kill Naix if no creeps are around.
Blink Dagger will help you escape even if Naix casted Open Wounds on you. It's a decent item on any hero. Simply blink away and run away.