It does not need to be all that threatening when nothing can touch it while it can still attack. 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.