The creature Ninjaman is trying to make never, at any time, has indestructible as it's listed in the rulebook. You have to deviate from the rules just to check for lethal damage as a state-based action. (The rules tell you to ignore those if a creature has indestructible.)
If it can be destroyed by lethal damage, it doesn't have indestructible. It's destructible. You can get the behavior Ninjaman wants by using Ogre Enforcer as precedent and not mention the indestructible keyword at all.