Actually, as an avid MMO fan(I am not goign to say I am great at them, but I do love them) I am taking into account the SAO universe's explained, hinted, and noted mechanics, as well as filling in the gaps with logical mechanics. In the SAO universe(the show, the books, etc.)players and monsters could become "crippled" by removing limbs, either by severed means, smashed means, etc. In example, during an episode, Kirito lost his left hand to a heavy sword slash, his health was dropped a degree, and then locked at that lowered number for a short while. Later that evening the "cripple" effect wore off and his hand was restored. The same will be taken into account for monsters and Bosses, though for bosses(primarily as they will not typically live long enough to worry about it) capping the lowered health number will not be implemented, rather the health will just drop a substantial sum, as it is a crippling injury, and the limb will be removed.