Without Kamui Kakashi can probably only kill tsunade with clone feints and raikiri. However, thats assuming he can even get to her. Once the sannin have their summons Kakashi's relevance is limited to sharingan genjutsu and clones.
Killer bee in bijuu mode should be able to handle the summons but there is Jiraiya with SM and swamp of the underworld, Tsunade's strength and healing ablity. Oro's rashomon gates might be a problem too I guess.
I say its sannin med-high difficulty.