As you probably know, Ninja only has Poison and Blind to soften up enemies and only Poison does damage. But what about actually giving them the skill of Corrode too? It won't directly drop HP, but as Ninjas are meant to be a stronger PvP class, Corrode would be a fitting skill for them to have?
As I doubt that Ninjas actually will get any other AoE that does damage, I think that it's a good idea with Corrode in their list of skills.

-Sigtyr