Correct, this is Limbus. Specifically Apollyon NW.
He is the boss for the third floor. My limbus group uses standard Genbu tactics to fight him. He will use Invincible. Pull and fight him on top of the HP/MP/2hr chest to restore all of your abilities instantly and he's not too hard. In fact, you shouldn't even need to pop the chest during the battle, but it's nice in case. We've never fought Adamantoise/Aspid, so I can't really compare.
The item chest will pop after defeating him. You can advance to the next floor without fighting him, however.