well im biased too, so we're even
onoki does have the ability to spam jinton, he used it a total of four times over the course of two seamless fights before being healed by tsunade, and he continued to use it afterwards. also, jinton has a 360 degree area of effect around the caster, as seen on page 10, chapter 575, destroying everything around him completely. this is most likely the effect of only the conical jinton, though, because when onoki prepared to fire his conical jinton at deidara, akatsuchi stopped him because it would destroy the island, implying that a direct hit with the beam is not necessary to kill. if onoki is smart enough to stay away from the ocean and any large body of water, which he most likely is, as he has more experience in fighting than most, i think he could win mid-high diff.
if kisame attempts to move in to steal chakra with samehada, onoki can create stone clones, and one of them will tag either samehada or kisame himself with ultra heavyweight jutsu, rendering his massive chakra supply useless.