Near the beginning of the fight, one of the first times Kenpachi slashed Gremmy, Yachiru (misspelt I know) stopped being a dessert long enough for her to escape. In order for Gremmy to 1-shot Kenny (uea that's the nickname I give him) he would have to fully concentrate on him to the point that the plateau he made would disappear and the vizards captains would "come back to life". Or so that's my opinion considering how OP Kenny is.
Now there's 2 problems with this.
1.) From the very beginning Gremmy underestimated everyone and everything to the poiny he said he's never truly had to fight anyone. Ergo, he has no experience in actual battle and simply 'played' with his enemies/victims. That superior attitude he has flawed him into thinking he could do what he wanted with Kenny.
2.) Now when he does realize his fault it's already too late. Kenny is slashing him over and over again he can barely concentrate on healing and blocking and dodging and whatnot. He missed his chance and therefore failed.
Wait one sec? What about the clones he made to 'amplify' his power? Well as we saw in the killer's high Kenny has already gotten to him and any actual thought aside from referring to 'killing' is gone. He even states it himself that he's never felt like this before and ypu clearly start to see his imagination start to get sloppy.
And it's actually true that all who have fought Kenny begin to get that high and any real thoughts are "slashed away" ( get it? no....ok). I wonder if that's an ability of his or just Kubo....