It has been done (Charles Oliveira has exceptional head movement on the ground) but it's very difficult and tends to be in concert with other forms of defence. Had his opponent been content to land mainly short hammerfists like he was at the start Kenshin probably would have been stuck there the entire round but he was posturing up and giving him the space to move.
The genuinely unrealistic thing is that Kenshin's slipping is so good when he's so inexperienced on the ground, but that just falls into the natural talent thing.
theres this japanese artist on twitter who draws a ton of fanart for red blue and honestly i thought it was their own original manga until i hit the translate button one time. anyway im glad this is getting translated again right when i start reading it. hope i get to see that guy with the buzz cut and his trainer soon