It does not need to be all that threatening when nothing can touch it while it can still attack.Interesting idea. But it isn't near as threatening as Zekrom-B and assist will frequently select one of the other moves on the Giratina set, meaning it won't work as effectively as it should.. This will surely cause a small rise in No Guard if this becomes at all popular though. I could see using No Guard Dialga, Latios, Arceus or even opposing Giratina.
Assist does not select the moves on the pokemon it is used on. No Guard should not hit on it either. Absolutely everything targetted on Giratina miss. Including self hit moves miss while under shadow force. So if you use assist then the next turn use swords dance it will miss(wtf). You can still use opponent targetted moves like attacks and curse(which is one of the only boosting moves that work).
It works way more effectively than it should.
Also kyurem-b with assist,flame charge, dragon tail, curse should be at least as good if not better than the Giratina set I posted.