And you are confusing stealth ninja kills with assassination.
Assassination is the targeted killing of a person, most of the time someone important in a public place.
Abraham lincoln was assassinated, even though his assassin was spotted by many.

Doing the job stealthly by sniper, or knife or blowing up the stage they are standing on doesn't really matter, the only thing matters is that its a suprise attack.

I know I would be fucking suprised if I saw a rocket coming at me for no reason, especially knowing by the time you realise you are going to blow the fuck up, it will already be too late ;o.

So yeah I like her bankai too, too heavily guarded? Just blow it the hell up.