Hirudora wasn't aiming at Kisame you say? Look at the attack properly [
You must be registered for see links
][
You must be registered for see links
]. It is going straight towards Kisame. Yes, it left him injured, but it didn't 'destroy' him. Kisame is, of course, less durable than Bee's v2 Jinchūriki form, as feats tell.
You can see the man's ribs. From a physical attack, his ribs were left exposed. Remembering that this is the same man who's body was still intact after Gai's trump card (Hirudora) was used.
An Amaterasu of that size would obviously do him damage in that state. The two techniques aren't comparable. One burns the other incinerates. The Hachibi took a direct hit from its own Bijūdama as it faced the Jūbi and it still lived to tell the tale. The point here is, although that v2 form has less fire power than the full Bijū, its shroud clearly offers the user greater protection and resistance to pain.
Exactly. You don't know.