Bosses who have a skull where their level should be, have the HP of something higher level (quite possibly level 500 like people say). But the actual boss isn't this level. Think of it this way: if a boss was level 500, nobody would be able to land a hit on him. He would dodge/parry/resist EVERYTHING. So, they fix this by granting the boss loads more HP while keeping the "succeptability" within reason. The magic level number you're looking for should just be referred to as "boss level" or something.

I don't know much about this subject, but this is what I've heard and it makes sense to me. So, I hope it's right and I hope it helps.